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Abstract:

An apparatus for performing electrodialysis at pressures greater than or
equal to the ambient pressure is described. The apparatus includes an
electrodialysis membrane stack and housing. The electrodialysis membrane
stack includes at least one electrodialysis cell. The electrodialysis
apparatus includes electrodes that apply voltage across the
electrodialysis stack. The housing pressurizes the electrodialysis stack
at a stack pressure. The housing includes a cell chamber that receives
the electrodialysis stack, the cell chamber including at least one
pressurization port communicating with the cell chamber such that a
portion of electrode solution is transmittable into a region of the cell
chamber outside the electrodialysis stack. A system for performing
electrodialysis at pressures greater than ambient pressure includes at
least two solution loops, an electrode solution loop, and an
electrodialysis apparatus operatively connected to the solution and
electrode solution loops that performs electrodialysis at a stack
pressure that is greater than ambient pressure.

Claims:

1. An electrodialysis apparatus comprising: an electrodialysis stack; a
pair of electrode grates, the electrodialysis stack substantially
interposed between the pair of electrode grates; a pair of electrodes,
the electrodialysis stack and the pair of electrode grates each
substantially interposed between the pair of electrodes; and a housing
configured to pressurize the electrodialysis stack at a stack pressure,
the housing including a cell chamber configured to receive the
electrodialysis stack, the cell chamber including at least one
pressurization port communicating with the cell chamber such that a
portion of an electrode solution is transmittable into a region of the
cell chamber outside the electrodialysis stack.

2. The electrodialysis apparatus of claim 1, the electrodialysis stack
comprising: two end gaskets, the two end gaskets each comprising one end
gasket opening and at least four end gasket apertures; and at least one
electrodialysis cell substantially interposed between the two end
gaskets, the at least one electrodialysis cell comprising: at least two
cell gaskets, the at least two cell gaskets each comprising one cell
gasket opening and at least two cell gasket apertures; and at least two
ion exchange membranes, the at least two ion exchange membranes each
comprising at least four membrane apertures, the at least two ion
exchange membranes alternating in an axial direction with the at least
two cell gaskets.

3. The electrodialysis apparatus of claim 2, the electrodialysis stack
further comprising at least one end ion exchange membrane substantially
interposed between the two end gaskets and the at least one
electrodialysis cell, the at least one end ion exchange membrane
comprising at least four membrane apertures.

4. The electrodialysis apparatus of claim 3, the electrodialysis stack
further comprising an additional cell gasket substantially interposed
between the two end gaskets and the at least one electrodialysis cell,
the additional cell gasket comprising one cell gasket opening and at
least two cell gasket apertures.

5. The electrodialysis apparatus of claim 4, wherein the two end gaskets,
the at least two cell gaskets, the at least two ion exchange membranes,
the at least one end ion exchange membrane, and the additional cell
gasket each further comprise at least one alignment feature such that
they are alignable and connectable with each other and with the housing.

6. The electrodialysis apparatus of claim 3, wherein the two end gaskets,
the at least two cell gaskets, the at least two ion exchange membranes,
and the at least one end ion exchange membrane each further comprise at
least one alignment feature such that they are alignable and connectable
with each other and with the housing.

7. The electrodialysis apparatus of claim 3, wherein the at least one end
ion exchange membrane is one of a bipolar exchange membrane, an anion
exchange membrane, or a cation exchange membrane.

8. The electrodialysis apparatus of claim 2, wherein the two end gaskets,
the at least two cell gaskets, and the at least two ion exchange
membranes all further comprise at least one alignment feature such that
they are alignable and connectable with each other and with the housing.

9. The electrodialysis apparatus of claim 2, wherein the at least two ion
exchange membranes are one of a bipolar exchange membrane, an anion
exchange membrane, or a cation exchange membrane.

11. The electrodialysis apparatus of claim 1, wherein the housing
comprises: a first axial support member and a second axial support member
configured to be operatively connected to form the cell chamber; and a
first reinforcing member operatively connected to the first axial support
member and a second reinforcing member operatively connected to the
second axial support member such that the first and second axial support
members are disposed between the first and second reinforcing members.

12. The electrodialysis apparatus of claim 11, the first and second axial
support members each comprising: an electrode solution inlet port
extending through the axial support members such that the electrode
solution is transmittable into the apparatus; an electrode solution
outlet port extending through the axial support members such that the
electrode solution is transmittable out of the apparatus; at least two
solution ports extending through the axial support members such that at
least two solutions are transmittable into or out of the apparatus; and
an electrode stem hole extending through the axial support members such
that an electrode stem is insertable through the axial support members.

13. The electrodialysis apparatus of claim 12, wherein the first and
second axial support members each further comprise an electrode recess
configured to receive one of the electrodes and one of the electrode
grates.

14. The electrodialysis apparatus of claim 13, wherein the electrode
recess comprises at least one electrode recess alignment feature
configured to substantially align the electrode grate in the electrode
recess.

15. The electrodialysis apparatus of claim 12, wherein the first axial
support member further comprises a cell recess configured to receive the
electrodialysis stack.

16. The electrodialysis apparatus of claim 15, wherein the first and
second axial support members further comprise at least one support member
alignment feature configured to substantially align the electrodialysis
stack with the first and second axial support members in an axial
direction and to substantially align the first and second axial support
members with each other.

17. The electrodialysis apparatus of claim 12, wherein the electrode
solution ports and the at least two solution ports are each substantially
bossed through a backside of the first and second axial support members.

18. The electrodialysis apparatus of claim 12, wherein the electrode
solution ports each have a first length that is less than a distance to
the cell chamber and a second length that is formed at an angle to the
first length.

19. The electrodialysis apparatus of claim 11, wherein the first and
second reinforcing members comprise: an electrode solution inlet port
hole extending through the reinforcing members such that an electrode
solution can pass through the reinforcing members; an electrode solution
outlet port hole extending through the reinforcing members such that an
electrode solution can pass through the reinforcing members; at least two
solution port holes extending through the reinforcing members such that
at least two solutions can pass through the reinforcing members; and an
electrode stem hole extending through the reinforcing members such that
an electrode stem is insertable through the reinforcing members.

21. The electrodialysis apparatus of claim 19, wherein the first and
second reinforcing members further comprise one or more of a stress
reduction cylinder and a plurality of flange adapters each substantially
connected to an outer side of the reinforcing members.

22. The electrodialysis apparatus of claim 1, the first and second
electrode grates comprising: an electrode grate face comprising a
plurality of exposure apertures configured to allow the electrode
solution to flow past the electrode grate and into the cell chamber; a
spacing rim formed substantially on an underside of the electrode grate
face and configured to raise the electrode grate face away from the
electrode; at least one grate solution port formed substantially within
the spacing rim and configured to allow the solution to flow under the
electrode grate face; and at least one flow diverting element formed
substantially on the underside of the electrode grate face and configured
to promote swirling of the electrode solution as it passes over the
electrode and into the cell chamber.

23. The electrodialysis apparatus of claim 22, wherein the first and
second electrode grates further comprise at least one electrode grate
alignment feature formed substantially on the spacing rim and configured
to align the electrode grate in the housing.

24. An electrodialysis stack comprising: two end gaskets, the two end
gaskets each comprising one end gasket opening and at least four end
gasket apertures; and at least one electrodialysis cell substantially
interposed between the two end gaskets, the at least one electrodialysis
cell comprising: at least two cells gaskets, the at least two cell
gaskets each comprising one cell gasket opening and at least two cell
gasket apertures; and at least two ion exchange membranes, the at least
two ion exchange membranes each comprising at least four membrane
apertures, the at least two ion exchange membranes alternating in an
axial direction with the at least two cells gaskets, wherein the two end
gaskets, the at least two cell gaskets, and the at least two ion exchange
membranes are all aligned in an axial direction such that at least two
solution loops are formed.

25. The electrodialysis stack of claim 24 further comprising at least one
end ion exchange membrane substantially interposed between the two end
gaskets and the at least one electrodialysis cell, the at least one end
ion exchange membrane comprising at least four membrane apertures, the at
least one end ion exchange membrane aligned in an axial direction with
the two end gaskets, the at least two cell gaskets, and the at least two
ion exchange membranes such that the at least one end ion exchange
membrane forms a part of the at least two solution loops.

26. The electrodialysis stack of claim 25 further comprising an
additional cell gasket substantially interposed between the two end
gaskets and the at least one electrodialysis cell, the additional cell
gasket comprising one cell gasket opening and at least two cell gasket
apertures, the additional cell gasket aligned in an axial direction with
the two end gaskets, the at least two cell gaskets, the at least two ion
exchange membranes, and the at least one end ion exchange membrane such
that the additional cell gasket forms a part of the at least two solution
loops.

28. An electrodialysis system for performing electrodialysis comprising:
at least two solution loops; an electrode solution loop; and an
electrodialysis apparatus operatively connected to the at least two
solution loops and the electrode solution loop and configured to perform
electrodialysis at a stack pressure that is greater than an ambient
pressure.

29. The electrodialysis system of claim 28, wherein one of the at least
two solution loops comprises a tank pressurized to a tank pressure that
is greater than an ambient pressure and less than the stack pressure such
that a gas is evolved in the tank at a pressure greater than ambient
pressure.

30. The electrodialysis system of claim 28, wherein one of the at least
two solution loops includes a means for selectively pressurizing the
solution loop to a loop pressure that is greater than an ambient pressure
and less than the stack pressure such that a gas is evolved from solution
at a pressure greater than ambient pressure.

[0004] Techniques for removing CO2 from streams of mixed gases, such
as removing the CO2 from power-plant flue-gas emissions or removing
CO2 from the atmosphere, typically involve a two-step process of
capture and regeneration. First, the gas is contacted with an aqueous
"pre-capture solution" that reacts with the CO2 gas in the mixed-gas
stream, "capturing" the CO2 into what is then referred to as a
"post-capture solution." Next, a stream of pure CO2 gas is
regenerated from this CO2-rich aqueous post-capture solution.
Various pre-capture solutions exist, with different solutions being
preferred depending on the concentration of CO2 in the mixed gas
source. For mixed gas streams with low concentrations of CO2--such
as the atmosphere with a CO2 concentration of 386 parts per million
(ppm) as of 2009 as shown by Dr. Pieter Tans, NOAA/ESRL
(www.esrl.noaa.gov/gmd/ccgg/trends)--aqueous hydroxide pre-capture
solutions such as potassium hydroxide (KOH) or sodium hydroxide (NaOH),
aqueous carbonate pre-capture solutions such as potassium carbonate
(K2CO3) or sodium carbonate (Na2CO3), or aqueous
bicarbonate pre-capture solutions such as potassium bicarbonate
(KHCO3) or sodium bicarbonate (NaHCO3) are likely candidates
for CO2 pre-capture solutions. The capture of CO2 gas into
these pre-capture solutions converts the original
hydroxide/carbonate/bicarbonate pre-capture solutions into a more acidic
post-capture solution consisting of a mixture of hydroxide (KOH or NaOH),
carbonate (K2CO3 or Na2CO3), and/or potassium
bicarbonate (KHCO3) or sodium bicarbonate (NaHCO3) post-capture
solutions, as examples.

[0005] Once the CO2 gas is captured from the mixed-gas stream into
the pre-capture solutions in the ionic forms CO3.sup.(2-) and/or
HCO3.sup.- to form the post-capture solutions, pure CO2 gas is
typically regenerated from the solution. The overall effect of this
process of capture and regeneration is the separation and concentration
of CO2 gas from a pre-separation mixed-gas stream with a relatively
low mole fraction of CO2 gas into a post-separation gas stream that
possesses a higher mole fraction of CO2 gas than the pre-separation
stream. Under the right conditions, the mole fraction of CO2 in the
post-separation stream may be unity, that is, the post-separation stream
may be a pure stream of CO2 gas. After capture and regeneration, the
post-separation gas can then be, for example, geologically sequestered,
or incorporated into useful products such as concrete, as shown by
Calera, Green Cement for a Blue Planet,
http://www.calera.com/index.php/technology/technology_vision/(last
visited Sep. 9, 2010); plastics, as shown by G. A. Olah et al., Beyond
Oil and Gas: The Methanol Economy, Wiley-VCH (2006); or liquid
hydrocarbon fuels, as shown by F. S. Zeman & D. W. Keith, Carbon Neutral
Hydrocarbons, Phil. Trans. R. Soc. A, 366, 3901-3918 (2008), and PARC,
Renewable Liquid Fuels,
http://www.parc.com/work/focus-area/renewable-liquid-fuels/(last visited
Sep. 9, 2010). Many of the possible uses of the regenerated CO2,
such as sequestration or reaction to liquid fuels, for example, require
the pressurization of the CO2 to pressures greater than 1 atm.

[0006] Bipolar membrane electrodialysis (BPMED) can be used to convert
aqueous salt solution into acids and bases without the addition of other
chemicals. A component of BPMED devices is ion exchange membranes used to
separate ionic species in solution when an electrical field is applied
across the membranes. Performing BPMED on certain solutions may create
gas bubbles adjacent to the membrane surface that can block ion transport
and reduce the effective membrane surface area, causing increased cell
resistance and localized "hot spots" of very high current density that
lead to shortened membrane lifetimes. As a result, commonly used input
and output solutions are selected so that they do not evolve significant
quantities of gas inside the membrane stack at ambient pressure, which
excludes an entire class of gas-evolving solutions from electrodialytic
treatment. Example embodiments address these and other disadvantages of
the conventional art.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is an exploded view of an embodiment of a high-pressure
electrodialysis apparatus.

[0008]FIG. 2 is an exploded view of an embodiment of a high-pressure
electrodialysis apparatus.

[0009]FIG. 3 is a front view of an interior face of a reinforcing member.

[0010]FIG. 4 is a front view of an exterior face of a reinforcing member.

[0011]FIG. 5 is a front view of an exterior face of a first axial support
member.

[0012]FIG. 6 is a front view of an interior face of a first axial support
member.

[0013]FIG. 7 is a front view of an interior face of a second axial
support member.

[0014] FIG. 8 shows an electrode.

[0015]FIG. 9 is a front view of an interior face of an electrode grate.

[0016]FIG. 10 is a front view of an exterior face of an electrode grate.

[0022]FIG. 16 is an exploded view of an embodiment of a high-pressure
electrodialysis apparatus.

[0023]FIG. 17 is a schematic of an embodiment of a high-pressure
electrodialysis system.

DETAILED DESCRIPTION

[0024] An embodiment of a high-pressure electrodialysis device has unique
advantages for the regeneration of CO2 gas from post-capture
solutions for use in, for example, liquid hydrocarbon fuel synthesis or
geological sequestration. The efficient synthesis of liquid hydrocarbon
fuel from CO2 gas and H2 gas reactants requires the gases to be
reacted at elevated temperatures and pressures, typically at least 200
degrees Celsius and 40 atm. The geological sequestration of CO2 gas
typically requires pressurization of the CO2 to 150 atm. What is
needed, then, is a means for regenerating CO2 from post-capture
solutions at elevated pressures so that the pressurized CO2 can then
be used in subsequent reactions, for example, hydrocarbon fuel synthesis
and geological sequestration.

[0025] In an embodiment, a high-pressure electrodialysis apparatus can
regenerate CO2 gas from aqueous post-capture solutions at any
pressure between the ambient pressure, for example 1 atm, and the
internal operating pressure, or stack pressure, of the electrodialysis
apparatus itself. In cases where the operating pressure and temperature
result in gaseous CO2 dissolved in solution after the
electrodialytic ion transfer, the CO2 regeneration is performed by a
gas/liquid separation of the CO2 from the solvent. In cases where
the operating pressure and temperature result in a solution/liquid
CO2 or solution/supercritical CO2 mixture after the
electrodialytic ion transfer, the CO2 regeneration is performed by
high-pressure electrodialysis followed by a liquid/liquid or
liquid/supercritical fluid separation of the CO2 from the solvent.
Embodiments of the apparatus are capable of taking as input an aqueous
post-capture solution and emitting as output a stream of pure or nearly
pure CO2 gas at an elevated pressure required for, for example,
efficient liquid hydrocarbon fuel synthesis or geological sequestration.

[0026] Currently available bipolar membrane electrodialysis (BPMED)
apparatuses include an electrodialysis stack made up of at least one
electrodialysis cell. The electrodialysis cell includes at least one
bipolar membrane (BPM) such that, when an electrical potential is applied
across the cell, the dissociation of water into hydrogen (H.sup.+) and
hydroxide (OH) ions occurs.

[0027] A BPM is formed from a cation-exchange layer laminated together
with an anion-exchange layer, as well as a junction layer between the
cation and anion layers. Water diffuses into the junction layer, reaching
the usual equilibrium with H.sup.+ and OH.sup.- ions according to its
dissociation constant. Typically, a BPM is oriented such that the
anion-exchange layer faces the anode (positive electrode) end of the
BPMED apparatus and the cation-exchange layer faces the cathode (negative
electrode) end of the apparatus. This orientation allows the OHions to be transported across the anion-exchange layer and the H.sup.+
anions to be transported across the cation-exchange layer when an
electrical potential is applied across the membrane stack.
Simultaneously, the constituent cations and anions of an input salt
solution are separated under the applied electrical potential via ion
exchange membranes--either anion exchange membranes (AEMs), cation
exchange membranes (CEMs), or some combination of the two. The OH.sup.(H.sup.+) ions "produced" by the BPM then combine with the cations
(anions) separated from the input salt solution to produce basic (acidic)
output solutions containing the parent acid and base of the input salt.
Depending on the membrane configuration, a diluted salt solution may also
be produced as output in addition to the acid and base output solutions.

[0028] A BPMED membrane stack can have either a two-compartment or a
three-compartment configuration. In a two-compartment configuration,
adjacent membranes may alternate between BPM and AEM to form a membrane
stack of the form BPM, AEM, BPM, AEM, etc.; or adjacent membranes may
alternate between BPM and CEM to form a membrane stack of the form BPM,
CEM, BPM, CEM, etc. In a three-compartment cell, adjacent membranes may
cycle from BPM to AEM to CEM, forming a membrane stack of the form BPM,
AEM, CEM, BPM, AEM, CEM, etc. Electrodialysis without bipolar membranes
is also possible, and consists of a two-compartment configuration formed
by an alternating series of AEM and CEM to form a membrane stack of the
form AEM, CEM, AEM, CEM, AEM, etc.

[0029] As mentioned above, gas bubbles inside commercially available BPMED
systems result in gas bubbles adjacent to the membrane surface that can
block ion transport in this region and reduce the effective membrane
surface area, resulting in increased resistance and localized "hot spots"
of very high current density that lead to shortened membrane lifetimes.
As a result, commonly used input and output solutions are selected so
that they do not evolve significant quantities of gas inside the membrane
stack at ambient pressure. This excludes an entire class of gas-evolving
solutions from electrodialytic treatment.

[0030] In an embodiment, these challenges are overcome by operating an
electrodialysis apparatus at high pressure, i.e., above ambient pressure.
In an embodiment, the operating pressure is sufficiently high, given
other process conditions, that a gas that would typically evolve out of
solution into the membrane stack at ambient pressure instead remains
dissolved in solution, preventing gas bubble evolution inside the
membrane stack itself. In an embodiment, the pressure of the solution
containing the dissolved gas can be reduced downstream of the membrane
stack so that the dissolved gas evolves out of solution and can be
collected for other uses, if desired. In an embodiment, the pressure to
which the solution is reduced can be any pressure less than the operating
pressure of the membrane stack. In this way, the gas can be evolved at
pressures greater than ambient pressure, and can also be evolved at
pressures less than ambient pressure, if desired. Electrodialysis
apparatuses according to certain embodiments may allow, for example, for
energy-efficient, high-rate concentration of CO2 in a compact,
reliable unit from the aqueous carbonate/bicarbonate capture solutions to
pure or nearly pure CO2 gas suitable, for example, to sequester, to
incorporate into useful products, or to react with other inputs to
synthesize liquid hydrocarbon fuels.

[0031] In cases where the operating pressure and temperature result in a
gas/liquid output solution after the electrodialytic ion transfer, the
gas regeneration is performed by a gas/liquid separation of the gas from
the solvent. In cases where the operating pressure and temperature result
in liquid/liquid or liquid/supercritical fluid output solution after the
electrodialytic ion transfer, the regeneration is performed by a
liquid/liquid or liquid/supercritical fluid separation of the liquid or
supercritical fluid from the solvent.

[0032] In some embodiments, a gas is absorbed into aqueous solution at
some pressure plow. High-pressure electrodialysis is then performed
on the solution, and then the same gas is regenerated at a pressure
phigh with phigh>plow. In this way, the embodiments may
be used as gas pressurization devices capable of replacing inefficient
mechanical compressors.

[0033] In an embodiment, an electrodialysis apparatus is designed to
operate at pressures greater than or equal to ambient pressure, ambient
pressure being defined as the pressure naturally occurring in the
environment surrounding the apparatus. At an elevation of sea-level, the
ambient pressure is typically around 1 atm, or 101.325 kPa. For example,
in an embodiment, the high-pressure electrodialysis apparatus can operate
at pressures up to and including 20 atm. In an embodiment, the
high-pressure electrodialysis apparatus can operate at pressures above 20
atm. Accordingly, in an embodiment a high-pressure electrodialysis system
may include positive displacement pumps to achieve high pressure in a
single stage, with accumulators to smooth the flow if needed. It may also
include solution tanks designed to withstand pressures of 20 atm and
greater, allowing the system to evolve gas at any pressure between the
ambient pressure and the membrane stack operating pressure, as well as
reducing pumping losses in all loops via external pressurization. In this
manner, gas can be evolved such that the evolved gas itself is at a
pressure above ambient pressure. Various types of gases may be produced
in this manner, depending on the inputs into the system. As examples,
CO2 gas can be produced when aqueous carbonate or bicarbonate
solutions are input into the system; SO2 gas can be produced when
aqueous sulfite or bisulfate solutions are input into the system and made
more acidic via operation of the system; and NH3 gas can be produced
when aqueous ammonium solutions are input into the system and made more
basic via operation of the system. Producing gases at pressures greater
than ambient pressure can be advantageous for many possible uses of the
separated gas. For example, CO2 gas separated in this manner can be
used in sequestration or reaction to liquid hydrocarbon fuel. The actual
pressure in the evolution tanks can be controlled by a regulator. To
prevent large pressure differentials from occurring across the membranes,
potentially leading to membrane damage, the system can include
closed-loop computer control of both flow and pressure.

High-Pressure Electrodialysis Apparatus

[0034]FIG. 1 shows an exploded view of a high-pressure electrodialysis
apparatus according to an embodiment. The apparatus includes an
electrodialysis stack 101. The electrodialysis stack 101 includes at
least one electrodialysis cell 102, although the apparatus may be adapted
to receive any number of electrodialysis cells 102.

[0035] In an embodiment, the electrodialysis cell 102 includes a first
cell gasket 103 rotated to a first angle, a first ion exchange membrane
104, a second cell gasket 105 rotated to a second angle, and a second ion
exchange membrane 106. The first and second ion exchange membranes 104
and 106 may be either bipolar membranes, anion exchange membranes, or
cation exchange membranes, depending on the input solutions used and the
desired output solution. The electrodialysis cell 102 may, but need not
be, a two-compartment cell. For example, the electrodialysis cell 102 may
be instead a three-compartment cell, such as the three-compartment cell
shown in FIG. 12. One skilled in the art will recognize that whether a
two- or three-compartment cell is used depends on the desired chemistry.

[0036] In the two-compartment cell 102 shown in FIG. 1, the first angle is
the orientation that allows a first solution flowing into the apparatus
at the anode end 116 (i.e., the electrode held at a positive potential
relative to the other electrode), depicted by dashed line 118, to contact
the surface of the membranes adjacent to cell gasket 103; the first angle
also allows the first solution to flow out of the apparatus at the
cathode end 117 (i.e., the electrode held at a negative potential
relative to the other electrode), as depicted by dashed line 119. The
second angle is the orientation that allows a second solution flowing
into the apparatus at the anode end 116, depicted by dashed line 120, to
contact the surface of the membranes adjacent to cell gasket 105; the
second angle also allows the second solution to flow out of the apparatus
at the cathode end 117, as depicted by dashed line 121. The flows of the
first and second solutions may be reversed as desired.

[0037] For example, when cell gasket 103 is the first cell gasket of the
first electrodialysis cell 102 in an electrodialysis stack 101, the first
angle to which cell gasket 103 is rotated allows a first solution flowing
into the apparatus (dashed line 118) through an inlet port (not shown) at
the anode end 116 to contact the surface of end ion exchange membrane 112
and first ion exchange membrane 104; the first angle also allows the
first solution to flow out of the apparatus (dashed line 119) through
outlet port 107 at the cathode end 117. Correspondingly, the second angle
to which cell gasket 105 is rotated allows a second solution flowing into
the apparatus (dashed line 120) through an inlet port (not shown) at the
anode end 116 to contact the surface of first ion exchange membrane 104
and second ion exchange membrane 106; the second angle also allows the
second solution to flow out of the apparatus (dashed line 121) through
outlet port 108 at the cathode end 117. Where cell gasket 103 is the
first cell gasket of an n-cell electrodialysis cell 102 in an
electrodialysis stack 101 with n>1 electrodialysis cells 102, the
first angle to which cell gasket 103 is rotated allows solution flowing
into the apparatus to contact the surface of the second ion exchange
membrane 106 of the preceding electrodialysis cell 102 and the first ion
exchange membrane 104 of the electrodialysis cell 102 to which gasket 103
belongs. In this way, cell gaskets 103 and 105 define separate
solution-receiving compartments.

[0038] In an embodiment, the first and second solutions are flowed through
the electrodialysis stack 101 via a series of apertures and cell gasket
openings that are formed in the different elements of the stack. For
example, in an embodiment where the electrodialysis stack 101 includes
one two-compartment BPMED cell (for example, where membrane 104 is a
bipolar membrane and membrane 106 is either an anion exchange membrane or
a cation exchange membrane) interposed between two end ion exchange
membranes 112 and 114 and two end gaskets 111 and 115, with an additional
cell gasket 113 between the electrodialysis cell 102 and the second end
ion exchange membrane 114, the following series of features, proceeding
from the anode end 116 to the cathode end 117, align to form the first
solution loop, as depicted by dashed lines 118 and 119: 1) two end gasket
apertures 131 of the first end gasket 111, 2) two membrane apertures 132
of the first end ion exchange membrane 112, 3) the cell gasket opening
135 of the first cell gasket 103, 4) two membrane apertures 132 of the
first ion exchange membrane 104, 5) two gasket apertures 133 of the
second cell gasket 105, 6) two membrane apertures 132 of the second ion
exchange membrane 106, 7) the cell gasket opening 135 of the additional
cell gasket 113, 8) two membrane apertures 132 of the second end ion
exchange membrane 114, and 9) two end gasket apertures 131 of the second
end gasket 115. The following series of features, proceeding from the
anode end 116 to the cathode end 117, align to form the second solution
loop in the same stack, as depicted by dashed lines 120 and 121: 1) two
end gasket apertures 131 of the first end gasket 111, 2) two membrane
apertures 132 of the first end ion exchange membrane 112, 3) two gasket
apertures 133 of the first cell gasket 103, 4) two membrane apertures 132
of the first ion exchange membrane 104, 5) the cell gasket opening 135 of
the second cell gasket 105, 6) two membrane apertures 132 of the second
ion exchange membrane 106, 7) two gasket apertures 133 of the additional
cell gasket 113, 8) two membrane apertures 132 of the second end ion
exchange membrane 114, and 9) two end gasket apertures 131 of the second
end gasket 115. Similar alignments of features can be used in embodiments
that use three-compartment BPMED cells or electrodialysis cells that do
not employ bipolar membranes but only employ anion exchange membranes and
cation exchange membranes.

[0039] In an embodiment, the first solution, the flow of which is
represented by dashed lines 118 and 119, is a basic solution (pH>7)
that is made more basic upon passing through the operating
electrodialysis unit such that the compartment defined by the cell gasket
opening 135 of cell gasket 103 is a basic solution-receiving compartment.
The second solution, the flow of which is represented by dashed lines 120
and 121, is an acidic solution (pH<7) that is made more acidic upon
passing through the operating electrodialysis unit such that the
compartment defined by the cell gasket opening 135 of cell gasket 105 is
an acidic solution-receiving compartment. It will be appreciated that the
first solution can be an acidic and the second solution basic, depending
on which solutions are flowed into which ports.

[0040] In an embodiment of a two-compartment cell, the first and second
cell gaskets 103 and 105 alternate with the first and second ion exchange
membranes 104 and 106 such that a first cell gasket-first ion exchange
membrane-second cell gasket-second ion exchange membrane sandwich is
formed. The first ion exchange membrane 104 is substantially interposed
between the first and second cell gaskets 103 and 105, and the second
cell gasket 105 is substantially interposed between the first and second
ion exchange membranes 104 and 106.

[0041] In another embodiment, the electrodialysis cell is a
three-compartment BPMED cell instead (see, example, FIG. 12), where each
cell contains three membranes, one of which is a bipolar membrane, one of
which is an anion exchange membrane, and one of which is a cation
exchange membrane. The three-compartment cell includes a first cell
gasket, a first ion exchange membrane, a second cell gasket, a second ion
exchange membrane, a third cell gasket, and a third ion exchange
membrane. The first cell gasket is rotated to a first angle such that a
first solution flowed into the apparatus through an inlet port at the
anode end 116 contacts the surface of the membranes adjacent to the first
cell gasket. The first angle also allows the first solution to be flowed
out of the apparatus through an outlet port at the cathode end 117. The
second cell gasket is rotated to a second angle such that a second
solution flowed into the apparatus through an inlet port at the anode end
116 contacts the surface of the membranes adjacent to the second cell
gasket. The second angle also allows the second solution to be flowed out
of the apparatus through an outlet port at the cathode end 117. The third
cell gasket is rotated to a third angle such that a third solution flowed
into the apparatus through an inlet port at the anode end 116 contacts
the surface of the membranes adjacent to the third cell gasket. The third
angle also allows the third solution to be flowed out of the apparatus
through an outlet port at the cathode end 117. The flows of the first,
second, and third solutions may be reversed as desired.

[0042] In an embodiment, the electrodialysis stack 101 includes only one
three-compartment cell such that the first solution flowed into the
apparatus and into the solution-receiving compartment of the first cell
gasket contacts the surface of the end ion exchange membrane and the
first ion exchange membrane of the cell; the second solution flowed into
the apparatus and into the solution-receiving compartment of the second
cell gasket contacts the surface of the first and second ion exchange
membranes of the cell; and the third solution flowed into the apparatus
and into the solution-receiving compartment of the third cell gasket
contacts the surface of the second and third ion exchange membranes of
the cell. In an embodiment, the electrodialysis stack 101 includes more
than one three-compartment cell such that the first solution flowed into
the apparatus and into the solution-receiving compartment of the first
cell gasket of the nth cell contacts the surface of the third ion
exchange membrane of the preceding cell and the first ion exchange
membrane of the nth cell; the second solution flowed into the apparatus
and into the solution-receiving compartment of the second cell gasket of
the nth cell contacts the surface of the first and second ion exchange
membranes of the nth cell; and the third solution flowed into the
apparatus and into the solution-receiving compartment of the third cell
gasket of the nth cell contacts the surface of the second and third ion
exchange membranes of the nth cell. In this way, the first, second, and
third cell gaskets define separate solution-receiving compartments.

[0043] In an embodiment of a three-compartment cell, the
solution-receiving compartment of the first cell gasket receives a basic
solution that is made more basic upon passing through the operating
electrodialysis unit, the solution-receiving compartment of the second
cell gasket receives a salt solution that is diluted upon passing through
the operating electrodialysis unit, and the solution-receiving
compartment of the third cell gasket receives an acidic solution that is
made more acidic upon passing through the operating electrodialysis unit.
However, it will be appreciated that what the first, second, and third
solutions are depends on which solutions are flowed into which ports.

[0044] In an embodiment of a three-compartment cell, the first, second,
and third cell gaskets alternate with the first, second, and third ion
exchange membranes such that a first cell gasket-first ion exchange
membrane-second cell gasket-second ion exchange membrane-third cell
gasket-third ion exchange membrane sandwich is formed. The first ion
exchange membrane is substantially interposed between the first and
second cell gaskets, the second cell gasket is substantially interposed
between the first and second ion exchange membranes, the second ion
exchange membrane is substantially interposed between the second and
third cell gaskets, and the third cell gasket is substantially interposed
between the second and third ion exchange membranes.

[0045] In an embodiment of a cell--either a either two-compartment,
three-compartment, or non-BPMED cell--spacers (not shown) are used to
substantially fill the gasket openings of the end gaskets and the cell
gaskets. For example, in a two-compartment cell, spacers are used to
substantially fill the gasket openings 134 and 135 of the end gaskets 111
and 115 and cell gaskets 103, 105, and 113 respectively. The spacers may
be, for example, polypropylene mesh, such as, for example, XN-4820
netting made by Industrial Netting. In an embodiment, the spacers are cut
to a size that substantially fills the gasket openings. The spacers may
have a thickness that is approximately the same thickness as the end
gaskets 111 and 115 and cell gaskets 103, 105, and 113, for example,
0.031'', and may have a nominal hole size of approximately 0.1'' and an
open percent of approximately 76%. The spacers prevent the ion exchange
membranes 112, 104, 106, and 114 from touching each other. They also
optimize the mass transport of ions to the adjacent membrane faces by
distributing the flow of solutions across the adjacent membrane surfaces.

[0046] In an embodiment of a cell--either a either two-compartment BPMED,
three-compartment BPMED, or an electrodialysis cell without a bipolar
membrane--all of the cell gaskets of the cell, as well as the additional
cell gasket, are substantially the same, both in material and in shape,
size, and formation. Each cell gasket is rotated at an angle to its
neighboring cell gaskets. For example, in a two-compartment cell, the
first cell gasket 103 and second cell gasket 105 are rotated at
substantially 90 degree angles to each other. The rotation axis may be
defined by, for example, the axis passing straight through the middle of
the cell gasket opening.

[0047] In an embodiment, the apparatus is a high-pressure two-compartment
BPMED apparatus. The first ion exchange membrane 104 may be, for example,
a bipolar exchange membrane (BPM), and the second ion exchange membrane
106 may be, for example, an anion exchange membrane (AEM). In another
embodiment, the first ion exchange membrane 104 may be, for example, a
BPM, and the second ion exchange membrane 106 may be, for example, a
cation exchange membrane (CEM). In another embodiment, the apparatus is
not a BPMED apparatus, but is an electrodialysis apparatus with no
bipolar membranes; the first ion exchange membrane 104 may be, for
example, an AEM, and the second ion exchange membrane 106 may be, for
example, a CEM.

[0048] In an embodiment, the apparatus is a high-pressure
three-compartment BPMED apparatus. The first ion exchange membrane may
be, for example, a BPM; the second ion exchange membrane may be, for
example, an AEM; and the third ion exchange membrane may be, for example,
a CEM.

[0049] In an embodiment, the electrodialysis stack 101 also includes end
gaskets 111 and 115 and end ion exchange membranes 112 and 114. End ion
exchange membrane 112 is substantially interposed between the first cell
gasket 103 of the first electrodialysis cell 102 and the end gasket 111.
In an embodiment, the electrodialysis stack 101 also includes an
additional cell gasket 113 such that the end ion exchange membrane 114 is
substantially interposed between the additional cell gasket 113 and the
end gasket 115. End gaskets 111 and 115 include end gasket openings 134
defined to receive an electrode solution that is flowed into and out of
the apparatus. In an embodiment, the electrode solution is flowed into
the apparatus at both the anode 116 and cathode 117 ends through inlet
ports at each end, for example, port 109; received in the electrode
solution-receiving compartments defined by end gaskets 111 and 115;
flowed back out of the apparatus at both the anode 116 and cathode 117
ends through outlet ports at each end, for example, port 110; and
recombined in an electrode solution receiving tank operatively connected
to the apparatus such that electroneutrality of the electrode solution is
preserved.

[0050] In an embodiment, the electrodialysis stack 101 has the following
alignment of features proceeding from the anode end 116 to the cathode
end 117 along an axial direction: 1) end gasket 111, 2) end ion exchange
membrane 112, 3) first cell gasket 103, 4) first ion exchange membrane
104, 5) second cell gasket 105, 6) second ion exchange membrane 106, 7)
additional cell gasket 113, 8) end ion exchange membrane 114, and 9) end
gasket 115. The end ion exchange membranes 112 and 114 may be, for
example, CEMs; the first ion exchange membrane 104 may be, for example, a
BPM; and the second ion exchange membrane 106 may be, for example, an
AEM.

[0051] In an embodiment, the electrodialysis stack 101 includes end
gaskets 111 and 115 and end ion exchange membrane 112, but does not
include end ion exchange membrane 114 and additional cell gasket 113. For
example, in an electrodialysis stack with one two-compartment BPMED cell
composed of one BPM and one AEM, the electrodialysis stack 101 would have
the following alignment of features proceeding from the anode end 116 to
the cathode end 117 along an axial direction: 1) end gasket 111, 2) end
ion exchange membrane 112 (a CEM), 3) first cell gasket 103, 4) first ion
exchange membrane 104 (a BPM), 5) second cell gasket 105, 6) second ion
exchange membrane 106 (an AEM), and 7) end gasket 115.

[0052] In an embodiment that uses a single three-compartment cell, the
electrodialysis stack 101 includes end gaskets 111 and 115 and end ion
exchange membrane 112, but does not include end ion exchange membrane 114
and additional cell gasket 113. For example, the electrodialysis stack
101 may have the following alignment of features proceeding from the
anode end 116 to the cathode end 117 along an axial direction: 1) end
gasket 111, 2) end ion exchange membrane 112 (a CEM), 3) first cell
gasket 103, 4) first ion exchange membrane 104 (a BPM), 5) second cell
gasket 105, 6) second ion exchange membrane 106 (an AEM), 7) third cell
gasket (see cell gasket 1205, FIG. 12), 8) third ion exchange membrane
(see membrane 1206, FIG. 12) (a CEM), and 9) end gasket 115. It will be
recognized that different variations of electrodialysis stacks 101 are
possible, depending on the input solutions used and the desired output
solution.

[0053] By including at least one electrodialysis cell 102 and, in an
embodiment, end ion exchange membranes 112 and 114, the electrodialysis
stack 101 is configured to perform an electrodialysis operation on a
solution at a stack pressure at or above the ambient pressure surrounding
the apparatus. The stack pressure is the internal operating pressure of
the apparatus; it is any pressure equal to or greater than ambient
pressure. In an embodiment, the stack pressure is determined by the
pressure of the operating solutions, i.e., the electrode solution and the
first, second, and any third solutions. For example, in an
electrodialysis apparatus where an electrode solution and a first and
second solution are flowed into the apparatus, the first, second, and
electrode solutions are each pressurized to the same pressure; the stack
pressure, then, is whatever pressure the first, second, and electrode
solutions are pressurized to. Making the pressures of the operating
solutions equal inside the membrane stack ensures no pressure
differentials across the membranes. In another embodiment, the operating
solutions may be at different pressures. In an embodiment, the stack
pressure is a pressure between ambient pressure and 20 atm. In an
embodiment, the stack pressure is a pressure greater than or equal to 20
atm.

[0054] For example, in an embodiment the electrodialysis stack 101
includes one or more two-compartment electrodiaysis cells 102 configured
with a BPM and an AEM. An acidic solution is flowed into the apparatus at
the anode end 116 through an acid inlet port (not shown), as depicted by
dashed line 120. A basic "post-capture" carbonate/bicarbonate solution is
also flowed into the apparatus at the anode end 116 through a base inlet
port (not shown), as depicted by dashed line 118. The BPM(s) of the
electrodialysis cell(s) 102 dissociate water under an applied voltage
such that OH.sup.- ions are transported across the BPM toward the anode
end 116 and H.sup.+ ions are transported across the BPM toward the
cathode end 117. HCO3.sup.- or CO3.sup.(2-) ions from the
post-capture basic solution are transported across the AEM(s) toward the
anode end 116 and into the acidic solution such that CO2 gas is
evolved from the acidic solution at sufficiently low pressures. It will
be appreciated that other types of aqueous solutions may alternatively be
used such that other types of gas are evolved. For example, SO2 gas
can be produced when aqueous sulfite or bisulfate solutions are input
into the system and made more acidic via operation of the system, and
NH3 gas can be produced when aqueous ammonium solutions are input
into the system and made more basic via the operation of the system.

[0055] In an embodiment for which gas is produced when the input solution
is made more acidic (such as CO2 from carbonate and bicarbonates and
SO2 from sulfites and bisulfites), the stack pressure, defined as
the pressure of the electrodialysis stack 101, is kept sufficiently high
given operating conditions such as, for example, input solution
composition and current density, that gas does not evolve from the acid
solution inside the apparatus but instead remains dissolved in the
aqueous acid solution. In an embodiment, after the acid solution is
flowed out of the apparatus, the pressure of the acid solution can be
lowered to a sufficiently low pressure given the operating conditions
that the gas evolves out of the acidic solution. In an embodiment, the
electrodialysis system includes tanks pressurized at a tank pressure that
is above ambient pressure but less than the stack pressure such that the
evolved gas is itself at a pressure above ambient pressure. In an
embodiment, CO2 gas at a pressure above ambient pressure can be
evolved. Producing CO2 gas at pressures greater than ambient
pressure can be advantageous for many possible uses of the separated
CO2 gas, including sequestration and reaction to liquid hydrocarbon
fuel. The tank pressure may be any pressure above ambient pressure but
less than the stack pressure. In an embodiment, the stack pressure is a
pressure between 1 atm and 20 atm. In an embodiment, the stack pressure
is equal to or greater than 20 atm.

[0056] The electrodialysis apparatus also includes a first electrode (not
shown) at the anode end 116 and second electrode 122 at the cathode end
117 of the apparatus such that a voltage can be applied across the
electrodialysis stack 101. The electrodialysis stack 101 is substantially
interposed between the first electrode and second electrode 122. In an
embodiment, the apparatus also includes a first electrode grate 123
interposed between the first electrode and the electrodialysis stack 101,
and a second electrode grate (not shown) interposed between the second
electrode 122 and the electrodialysis stack 101.

[0057] The electrodialysis apparatus also includes a housing comprising
two axial support members 126 and 127 and two reinforcing members 124 and
125. In an embodiment, the first axial support member 126 and the second
axial support member 127 are configured to be operatively connected
together to form a cell chamber. The cell chamber receives the
electrodialysis stack 101. The cell chamber also provides a "pressure
basket" around the electrodialysis stack 101 when the apparatus is
pressurized. In an embodiment, the first axial support member 126
includes at least one pressurization port 128 that communicates with the
cell chamber such that a portion of an electrode solution flowed into the
electrode chamber via electrode solution ports in the first axial support
member 126 (not shown) is transmittable into a region of the cell chamber
outside the electrodialysis stack 101.

[0058] In an embodiment, the axial support members 126 and 127 have bossed
ports that pass through radiused port holes in the reinforcing members
124 and 125 to allow either first, second, or electrode solution,
depending on the port, to enter and exit the electrodialysis apparatus.
In embodiments that include one or more three-compartment BPMED cells,
there is an additional bossed port and radiused port hole through which
the third solution can enter and exit the apparatus. Because the bossed
ports can be slid through the radiused port holes in the reinforcing
members 124 and 125, assembly of the axial support members 126 and 127 to
the reinforcing members 124 and 125 can be accomplished without welding.
Further, the bossed ports allow solutions to pass through the reinforcing
members 124 and 125 without contacting the reinforcing members 124 and
125 so that the reinforcing members 124 and 125 do not form part of the
flow channel. This allows different materials to be used to form the
axial support members 126 and 127 and the reinforcing members 124 and
125. For example, in an embodiment the axial support members 126 and 127
may be formed from PVC, whereas the reinforcing members 124 and 125 may
be formed from 4340 steel. Because the bossed ports of the axial support
members 126 and 127 allow fluids to pass through the reinforcing members
124 and 125 without contacting the reinforcing members 124 and 125,
corrosion of the steel of the reinforcing members by the fluids is
avoided. The bossed ports can be connected, via a seal such as a static
o-ring face seal, with flange adapters 129 to which fluid pumping hoses
may be attached.

[0059] In an embodiment, the apparatus also includes two stress reduction
cylinders 130, each one attached to the outside face of one of the
reinforcing members 124 and 125. In another embodiment, stress reduction
cylinders 130 are not included.

[0060]FIG. 2 shows an exploded view of another embodiment of a
high-pressure electrodialysis apparatus. The housing includes two axial
support members 203 (one not shown), two reinforcing members 204 and 205,
and two sleeve members 206 and 207. The axial support members do not
include protruding bosses for the ports. The axial support members are
attached to the reinforcing members 204 and 205 by, for example, welding.
The reinforcing members 204 and 205 have port holes that align with the
ports of the axial support members such that the reinforcing members 204
and 205 form part of the flow channel. As such, the reinforcing members
204 and 205 need to be compatible with the reagents, i.e., made of a
material that can withstand corrosion, for example, 316 stainless steel.
Because stainless steel is not as strong as, for example, 4340 steel, the
reinforcing members 204 and 205 may be thicker than reinforcing members
made from other steels.

[0061] The axial support members may be operatively connected together by
the sleeve members 206 and 207. The sleeve members 206 and 207 are a
first cylinder and a second cylinder, respectively. In an embodiment, the
first and second cylinders 206 and 207 are concentric steel cylinders
with a sliding seal. The cylinders allow accommodation of an
electrodialysis stack 201 with a varying number of cells 202 and provide
a "pressure basket" around the gasketed membranes. This puts less stress
on the gasket seals because pressure in the cell chamber is essentially
the same as in the electrodialysis stack, the cell chamber pressure being
supplied by a small diversion off one of the solution loops through a
pressurization port.

[0062] The apparatus also includes an electrodialysis stack 201. The
electrodialysis stack 201 may include one or electrodialysis cells 202,
such as those described above with respect to FIG. 1. In an embodiment,
the electrodialysis stack 201 includes one or more cells 202 configured
to perform two-compartment BPMED. In another embodiment, the
electrodialysis stack 201 includes one or more cells 202 configured to
perform electrodialysis without bipolar membranes. In another embodiment,
the electrodialysis stack 201 includes one or more cells 202 configured
to perform three-compartment electrodialysis. Embodiments of
electrodialysis stack 201 and electrodialysis cell 202 are substantially
the same as embodiments of electrodialysis stack 101, described above
with respect to FIG. 1. The apparatus also includes two electrodes 208
(one not shown), and, in an embodiment, two electrode grates (not shown).

[0063] The various features of the embodiment apparatuses discussed above
with respect to FIGS. 1-2 will be discussed in more detail below with
respect to FIGS. 3-15.

[0064] Turning to FIG. 3, a substantially planar view of the interior face
of a first reinforcing member according to an embodiment is shown. The
interior face of a second reinforcing member according to an embodiment
is substantially the same except with respect to the relative positioning
of the solution port holes. Therefore, the interior face of the second
reinforcing member will not be separately discussed; differences will be
noted below.

[0065] In an embodiment, the first reinforcing member includes electrode
solution port holes 301 and 302 to allow the bossed electrode solution
ports of the first axial support member (see FIG. 5) to pass through the
first reinforcing member. In an embodiment where the ports of the first
axial member are not bossed (see FIG. 2), the port holes 301 and 302
align with the electrode solution ports of the first axial support member
instead. In an embodiment, electrode solution port hole 301 corresponds
to an electrode solution inlet port of the first axial support member,
and electrode solution port hole 302 corresponds to an electrode solution
outlet port of the first axial support member. In an embodiment,
electrode solution port hole 301 corresponds to an electrode solution
outlet port of the first axial support member, and electrode solution
port hole 302 corresponds to an electrode solution inlet port of the
first axial support member.

[0066] In an embodiment, the electrode solution port holes 301 and 302 are
formed in the first reinforcing member such that they are substantially
directly across from each other. However, any other arrangement that
allows for the bossed electrode solution ports of the first axial support
member to pass through the first reinforcing member, or, in an
alternative embodiment, for the electrode solution ports of the first
axial support member to align, can be used. For example, in an embodiment
first axial support member with bossed electrode solution ports not
substantially directly across from each other, the electrode solution
port holes 301 and 302 also would not be substantially directly across
from each other. Instead, they would be formed in whatever positions
correspond to where the bossed electrode solution ports are formed on the
first axial support member.

[0067] The first reinforcing member also includes a first solution port
hole 303 and a second solution port hole 304 to allow the bossed first
and second solution ports of the first axial support member (see FIG. 5)
to pass through the first reinforcing member. In an embodiment where the
ports of the first axial member are not bossed (see FIG. 2), the port
holes 303 and 304 align with the first and second solution ports of the
first axial support member instead. In an embodiment, the first and
second solution port holes 303 and 304 are formed substantially on either
side of the electrode solution port hole 301. However, any other
arrangement that allows for the bossed first and second solution ports of
the first axial support member to pass through the first reinforcing
member, or, in an alternative embodiment, for the electrode solution
ports of the first axial support member to align, can be used. For
example, in an embodiment first axial support member with bossed first
and second solution ports not substantially on either side of the bossed
electrode solution port that corresponds to the electrode solution port
hole 301, the first and second solution port holes 303 and 304 also would
not be substantially on either side of the electrode solution port hole
301. Instead, they would be formed in whatever positions correspond to
where the bossed first and second solution ports are formed on the first
axial support member. Further, it will be appreciated that the first and
second solution port holes 303 and 304 may be in the opposite position
relative to each other to that shown, i.e., the first solution port hole
303 could be instead formed where the second solution port hole 304 is
formed, and vice versa.

[0068] In an embodiment, the first solution is an acidic solution and the
second solution is a basic solution such that an acidic solution inlet
port of the first axial support member passes through the first solution
port hole 303, and a basic solution inlet port of the first axial support
member passes through the second solution port hole 304. In another
embodiment, the first solution is a basic solution and the second
solution is an acidic solution such that a basic solution inlet port of
the first axial support member passes through the first solution port
hole 303, and an acidic solution inlet port of the first axial support
member passes through the second solution port hole 304. The solution
type (acidic or basic) that flows through the ports that pass through
solution port holes 303 and 304 depends on the orientation of the cell
gaskets (see FIG. 1) and the relative polarity of the electrodes (see
FIG. 1) in the electrodialysis stack. For example, in an embodiment of
the apparatus shown in FIG. 1 where 116 is the anode (relatively
positive) end, the first ion exchange membrane 104 is a BPM, and the
second ion exchange membrane 106 is an AEM, cell gasket 103 defines a
base-receiving compartment and cell gasket 105 defines an acid-receiving
compartment. The first solution would be an acidic solution such that the
first solution port hole 303 would correspond to an acidic solution inlet
port of the first axial support member of anode end 116, and the second
solution would be a basic solution such that the second solution port
hole 304 would correspond to a basic solution inlet port of the first
axial support member of anode end 116. In an embodiment, the direction of
the solution flow through the apparatus can be reversed, i.e., the
solution inlet ports that pass through the first and second solution port
holes 303 and 304 can become solution outlet ports instead, and the
solution type associated with that port hole 303 or 304 will not change.

[0069] For the second reinforcing member (not shown) of an embodiment
apparatus, the relative positions of the electrode solution port holes
may be opposite to those shown in FIG. 3. For example, if the reinforcing
member shown in FIG. 3 were a corresponding second reinforcing member
instead, the electrode solution port hole 301 may correspond to an
electrode solution outlet port on the second axial reinforcing member,
and the electrode solution port hole 302 may correspond to an electrode
solution inlet port on the second axial reinforcing member. However, it
is not necessary that the relative positions of the electrode solution
port holes of the first and second reinforcing members be opposite each
other, and in some embodiments the positioning may be the same.

[0070] For the second reinforcing member (not shown) of an embodiment
apparatus, the relative positions of the first and second solution port
holes may be opposite to those shown in FIG. 3. For example, if the
reinforcing member shown in FIG. 3 were a corresponding second
reinforcing member instead, in an embodiment the first solution port hole
303 may correspond to a basic solution outlet port of the second axial
reinforcing member, rather than an acidic solution inlet port like the
first reinforcing member does. The second solution port hole 304 may
correspond to an acidic solution outlet port of the second axial
reinforcing member, rather than a basic solution inlet port like the
first reinforcing member does.

[0071] In an embodiment of a high-pressure electrodialysis apparatus that
uses one or more three-compartment BPMED cells, both the first and second
reinforcing members include an additional, third solution port hole (not
shown). The third solution port hole is formed in a position that
corresponds with the third solution port of the corresponding axial
support member. In an embodiment, the third solution may be, for example,
a salt solution that is diluted upon passing through the operating
electrodialysis unit.

[0072] The first reinforcing member also includes an electrode stem hole
305 to allow the stem of the first electrode to pass through the first
reinforcing member. The electrode stem hole 305 may be formed, for
example, substantially in the center of the first reinforcing member.

[0073] The first reinforcing member also includes at least one reinforcing
member connection feature 306. The reinforcing member connection features
306 may be, for example, holes formed near the edges of the first
reinforcing member and passing through the first reinforcing member such
that a connecting feature (not shown) may be passed therethrough. The
connecting feature may be, for example, dowel pins. Alternatively, a
bolted connection may be used, for example, 10 mm, class 8.8 Hex head cap
screws. A bolted connection has a safety factor of about 3.6 at a cell
pressure of 20 atm, given the materials listed in previously described
embodiments of the various elements of an apparatus. The reinforcing
member connection features 306 allow the first reinforcing member to be
coupled to the second reinforcing member in such a way that the first and
second reinforcing members are substantially aligned adjacent to each
other. When the first and second reinforcing members are so coupled, a
compressive force is exerted along the axial direction, allowing the
first and second axial support members to be mated, as will be discussed
further with regard FIGS. 6-7.

[0074] Although the reinforcing member connection features 306 in FIG. 2
are depicted as holes, it will be appreciated that any shape that
facilitates alignment and application of a compressive force can be used.
Further, the reinforcing member connection features 306 may be formed
substantially at the edges of the first reinforcing member, for example
as notches, such that they are not enclosed on every side by the body of
the first reinforcing member.

[0075] The reinforcing members provide strength to the electrodialysis
apparatus. For example, when a stack pressure of 10 atm is applied to the
apparatus, there is approximately 4.4 metric tons of force acting to push
the apparatus apart in the axial direction. At a stack pressure of 20
atm, this doubles to nearly 9 metric tons (-20,000 lbf). In an
embodiment, the reinforcing members are formed from high strength
materials, for example 4340 steel, in order to withstand these pressures.
In an embodiment, the reinforcing member is formed from 4340 chrome
molybdenum steel in the annealed state that is approximately 0.75 inches
thick and coated with a corrosion-inhibiting paint. In an embodiment,
stress concentrations in the reinforcing members are reduced by radiusing
the electrode solution port holes 301 and 302, the first and second
solution port holes 303 and 304, and the electrode stem hole 305. In an
embodiment, non-load bearing portions of the reinforcing members can be
clipped to reduce the weight of the reinforcing member. In an embodiment,
the mass of the assembled apparatus is approximately 35 kg.

[0076]FIG. 4 shows a substantially planar view of the exterior face of a
first reinforcing member according to an embodiment. The planar view of
the exterior face of a second reinforcing member according to an
embodiment is substantially the same except with respect to the relative
positioning of the solution port holes. Therefore, the exterior face of
the second reinforcing member will not be separately discussed;
differences will be noted below. Further, the elements 401-406 in FIG. 4
are the same as elements 301-306 in FIG. 3 except that they are the
elements as seen from the exterior face, rather than the interior face,
of a first reinforcing member. Therefore, elements 401-406 will not be
discussed further except that differences will be noted below.

[0077] In an embodiment of a high-pressure electrodialysis apparatus that
uses one or more three-compartment BPMED cells, both the first and second
reinforcing members include an additional, third solution port hole (not
shown) formed in a position that corresponds with the third solution port
of the corresponding axial support member.

[0078] In an embodiment, the electrode solution port holes 401 and 402 and
the first and second solution port holes 403 and 404 are not radiused on
the exterior face of the first reinforcing member. In an embodiment, the
electrode stem hole 405 is also not radiused on the exterior face of the
first reinforcing member.

[0079] The first reinforcing member also includes at least one reinforcing
member connection feature 406. In an embodiment, the first reinforcing
member also includes a plurality of flange adapter mount holes 407
surrounding each port hole 401-404. In an embodiment, the flange adapter
mount holes 407 are formed only on the exterior face of the first
reinforcing member and do not pass through the first reinforcing members.
The flange adapter mount holes 407 are formed in a number and an
arrangement that substantially aligns with mount holes on the flange
adapters themselves (not shown). Flange adapters are mounted over the
port bosses of the axial support member (see FIG. 5) that protrude
through the port holes of the reinforcing member when the axial support
member and reinforcing member are coupled together. The flange adapter
mount holes 407 allow the flange adapters (not shown) to be mounted over
the protruding port bosses and attached to the first reinforcing member.

[0080] In an embodiment, the first reinforcing member includes a stress
reduction cylinder 408. The stress reduction cylinder 408 may be attached
to the first reinforcing member by, for example, welding. The stress
reduction cylinder 408 is a capped cylinder provided to reduce stress on
the reinforcing members when the electrodialysis apparatus operates at a
pressure above ambient pressure.

[0081] For example, a simulated stress analysis was conducted using
Solidworks FEA simulation tool. A reinforcing member formed from 4340
steel in the annealed state with a yield strength of approximately 4640
atm (68,200) psi was used. Pressure was applied to the interior face of
the reinforcing member where the axial support member contacts the
reinforcing member. Fixed restraints were placed at the reinforcing
member connection features. Because a fixed restraint was used, the
simulation showed higher stresses at these points than would be
experienced with, for example, a bolted connection; stress oddities at
these locations may be interpreted as a modeling artifact. Stress at both
10 atm (-150 psi) and 20 atm (-300 psi) was considered. The maximum
stress was always near the electrode stem hole.

[0082] The simulation was run both with and without a stress reduction
cylinder attached to the reinforcing member. At 10 atm (-150 psi), the
stress near the electrode stem hole in the simulation with the stress
reduction cylinder was approximately 530 atm (7800 psi), with a yield
safety of 8.7; the deflection near the electrode stem hole was 0.15 mm.
For the simulation without the stress reduction cylinder at 10 atm, the
stress near the electrode stem hole was approximately 676 atm (9935 psi),
with a yield safety of 6.86; the deflection near the electrode stem hole
was 0.175 mm. At 20 atm (300 psi), the stress near the electrode stem
hole in the simulation with the stress reduction cylinder was
approximately 1536 atm (22,580 psi), with a yield safety of 3; the
deflection near the electrode stem hole was 0.31 mm. For the simulation
without the stress reduction cylinder at 20 atm, the stress near the
electrode stem hold was approximately 1878 atm (27,600 psi), with a yield
safety of 2.5; the deflection near the electrode stem hole was 0.35 mm.

[0083] Because the yield safety factor without the stress reduction
cylinder is still 2.5 at 20 atm, embodiments that do not include the
stress reduction cylinder 408 can be used up to pressures of around 20
atm. Embodiments that do use a stress reduction cylinder will allow
operation at pressures greater than 20 atm, depending on the materials
used for the various parts of the apparatus. Other embodiments may not
include the stress reduction cylinder 408 but still be capable of
operating above 20 atm, depending on the materials used for the various
parts of the apparatus. In some instances, not including a stress
reduction cylinder 408 avoids having to weld the stress reduction
cylinder 408 to the reinforcing member, which avoids delay during
preheating, cooling, and heat treatment. In some instances, this also
avoids possible warpage, avoiding the need for additional corrective
work.

[0084] Although the simulation only considered pressures of 10 atm and 20
atm, in an embodiment the reinforcing members are designed to withstand
stack pressures greater than 20 atm. It will be appreciated that any
stack pressure, including pressures above 20 atm, can be used so long as
the yield safety factor is sufficiently high. For example, a yield safety
factor of at least 2 may be sufficient, depending on operating solutions
and conditions.

[0085] Although the embodiment first reinforcing members shown in FIGS.
3-4 are substantially octagonal in shape, it will be appreciated that
other shapes may be used for both the first and second reinforcing
members. For example, the first and second reinforcing members may be
substantially rectangular in shape, such as a square, or rounded, such as
a circle.

[0086] Turning to FIG. 5, a substantially planar view of the exterior face
of a first axial support member according to one embodiment is shown. The
planar view of the exterior face of a second axial support member
according to one embodiment is substantially the same except with respect
to the relative positioning and characterization of the solution ports.
Therefore, the exterior face of the second reinforcing member will not be
separately discussed; differences will be noted below.

[0087] In an embodiment, the first axial support member includes electrode
solution ports 501 and 502. The electrode solution ports 501 and 502
allow the electrode solution to enter and exit the apparatus. The
electrode solution may be, for example, an aqueous KOH or NaOH solution.
In an embodiment, the electrode solution ports 501 and 502 are bossed,
i.e., they protrude from the exterior face of the first axial support
member. This allows the electrode solution ports 501 and 502 to pass
through the first reinforcing member (see FIGS. 3-4). In an embodiment,
electrode solution port 501 is an electrode solution inlet port, and
electrode solution port 502 is an electrode solution outlet port. In an
embodiment, electrode solution port 501 is an electrode solution outlet
port, and electrode solution port 502 is an electrode solution inlet
port. In an embodiment, the electrode solution ports 501 and 502 are
formed in the first axial support member such that they are substantially
directly across from each other. However, any other arrangement can be
used, so long as the electrode solution port holes of the first
reinforcing member (see FIGS. 3-4) are formed in positions that
substantially align with the electrode solution ports 501-502.

[0088] For the second axial support member (not shown) of an embodiment
apparatus, the relative positions of the electrode solution ports may be
opposite to those shown in FIG. 5. For example, if the axial support
member shown in FIG. 5 were a corresponding second axial support member
instead, the electrode solution port 501 may be an electrode solution
outlet port, and the electrode solution port 502 may be an electrode
solution inlet port. However, it is not necessary that the relative
positions of the electrode solution ports of the first and second
reinforcing members be opposite each other, and in some embodiments the
positioning may be the same.

[0089] The first axial support member also includes a first solution port
503 and a second solution port 504. The first solution port 503 of the
first axial support member may be a first solution inlet, allowing the
first solution to enter the apparatus; the first solution port of the
second axial support member may be a first solution outlet, allowing the
first solution to exit the apparatus. If the flow of the apparatus is
reversed, the first solution port 503 of the first axial support member
may instead be a first solution outlet, and the first solution port of
the second axial support member may instead be a first solution inlet.
The second solution port 504 of the first axial support member may be a
second solution inlet, allowing the second solution to enter the
apparatus; the second solution port of the second axial support member
may be a second solution outlet, allowing the second solution to exit the
apparatus. If the flow of the apparatus is reversed, the second solution
port 504 of the first axial support member may instead be a second
solution outlet, and the second solution port of the second axial support
member may instead be a second solution inlet.

[0090] In an embodiment, the first solution is an acidic solution and the
second solution is a basic solution such that an acidic solution passes
through the first solution port 503 and a basic solution passes through
the second solution port 504 of the first axial support member. In
another embodiment, the first solution is a basic solution and the second
solution is an acidic solution such that a basic solution passes through
the first solution port 503 and an acidic solution passes through the
second solution port 504 of the first axial support member. The solution
type (acidic or basic) that flows through the ports 503 and 504 depends
on the orientation of the cell gaskets, the ordering of the membranes,
and the relative polarity of the electrodes in the electrodialysis stack
(see FIG. 1). For example, in an embodiment of the apparatus shown in
FIG. 1 where the first ion exchange membrane 104 is a BPM and the second
ion exchange membrane 106 is an AEM such that cell gasket 103 defines a
base-receiving compartment and cell gasket 105 defines an acid-receiving
compartment, the first solution would be an acidic solution such that the
first solution port 503 would be an acidic solution inlet port on the
first axial support member (or outlet port if the flow were reversed) of
anode end 116, and the second solution would be a basic solution such
that the second solution port 504 would be a basic solution inlet port on
the first axial support member (or outlet port if the flow were reversed)
of anode end 116.

[0091] The first and second solution ports 503 and 504, as well as the
first and second solution ports of the second axial support member, may
be bossed, i.e., protrude from the exterior face of the support member.
This allows the first and second solution ports 503 and 504 to pass
through the reinforcing member (see FIGS. 3-4). In an embodiment, the
first and second solution ports 503 and 504 are formed substantially on
either side of the electrode solution port 501. However, any other
arrangement can be used, so long as it allows for the first and second
solution ports 503 and 504 to substantially align with their
corresponding membrane and gasket apertures (see FIG. 1) such that the
first and second solutions flow through the apparatus, and so long as the
first and second solution port holes of the first reinforcing member (see
FIGS. 3-4) are formed in positions that substantially align with their
respective first and second solution ports 503-504.

[0092] The bossed ports of the first axial support member create solution
passageways such that the solutions pass through the first reinforcing
member (see FIGS. 3-4) without contacting the first reinforcing member.
This allows the first axial support member and the first reinforcing
member to be formed from different materials. For example, in an
embodiment the first axial support member is formed from PVC for chemical
compatibility with the working solutions, whereas the first reinforcing
member is formed from steel, for example 4340 steel, for strength. This
avoids contact by potentially corrosive solutions with the first
reinforcing member. Further, this allows the first reinforcing member to
be thinner than it might otherwise be. For example, a first reinforcing
member made of PVC may have to be very thick to withstand the sorts of
pressures an embodiment electrodialysis apparatus may operate under (4.4
metric tons of force in the axial direction at 10 atm, nearly 9 metric
tons at 20 atm). In contrast, an embodiment first axial support member
formed from, for example, 4340 chrome molybdenum steel can be thinner
than it would be if made from PVC, for example, approximately 0.75 inches
thick for operation up to 20 atm with a safety factor of 2.5 without use
of a stress reduction cylinder.

[0093] For the second axial support member (not shown) of an embodiment
apparatus, the relative positions of the first and second solution ports
may be opposite to those shown in FIG. 5. For example, if the axial
support member shown in FIG. 5 were a corresponding second axial support
member instead, in an embodiment the first solution port 503 may be a
basic solution outlet port, rather than an acidic solution inlet port
like in the first axial support member does. If the flow were reversed,
first solution port 503 may be a basic solution inlet port. The second
solution port 504 may be an acidic solution outlet port, rather than a
basic solution inlet port like in the first reinforcing member. If the
flow were reversed, second solution port 504 may be an acidic solution
inlet port.

[0094] In an embodiment of a high-pressure electrodialysis apparatus that
uses one or more three-compartment BPMED cells, both the first and second
axial support members include an additional, third solution port (not
shown). The third solution port is formed in a position that corresponds
with the third solution port hole of the corresponding reinforcing
member. In an embodiment, the third solution may be, for example, a salt
solution that is diluted upon passing through the operating
electrodialysis unit. The third solution port allows a third solution to
enter or exit the apparatus, depending on the direction of the flow.

[0095] The first axial support member also includes an electrode stem hole
505 to allow the stem of the first electrode to pass through the first
axial support member. The electrode stem hole 505 may be formed, for
example, substantially in the center of the first axial support member.

[0096] In an embodiment, the first axial support member also includes
seals 506. The seals 506 aid the connection of the flange adapters (not
shown) to the ports 501-504 of the first axial support member. In an
embodiment, the seals may be received in a groove formed in the outer
face of the port boss. In another embodiment, the seal may not be
received in a groove. In an embodiment, each port 501-504 has a seal 506.
The seal 506 may be, for example, a Viton® fluoroelastomer o-ring.

[0097] The first axial support member also includes a first reinforcing
member contact area 507. The first reinforcing member contact area 507
comes substantially in contact with the interior face of the first
reinforcing member (see FIG. 3) when the electrodialysis apparatus is
assembled.

[0098]FIG. 6 shows a substantially planar view of the interior face of a
first axial support member according to one embodiment. In an embodiment,
the planar view of the interior face of the second axial support member
is not substantially the same. The interior face of the second axial
support member will be discussed below with respect to FIG. 7. The
elements 601-605 in FIG. 6 are the same as elements 501-505 in FIG. 5
except that they are the elements as seen from the interior face, rather
than the exterior face, of the first axial support member.

[0099] The first axial support member includes electrode solution ports
601 and 602. Although electrode solution port 602 is obscured in FIG. 6,
it will be appreciated that in an embodiment, electrode solution port 602
is substantially the same configuration as electrode solution port 601.
The electrode solution ports 601 and 602 allow the electrode solution to
enter and exit the apparatus. In an embodiment, the electrode solution
ports 601 and 602 are formed in the first axial support member such that
they are substantially directly across from each other. However, any
other arrangement can be used, so long as the electrode solution port
holes of the first reinforcing member (see FIGS. 3-4) are formed in
positions that substantially align with their respective electrode
solution ports 601-602.

[0100] In an embodiment, the electrode solution inlet and outlet ports 601
and 602 do not pass straight through the entire thickness of the first
axial support member. Rather, the electrode solution ports 601 and 602
pass from the exterior side (see FIG. 5) substantially straight through
only part of the thickness of the first axial support member, stopping
short of the cell recess 606, forming a "blind" hole at each port. From
there, an angled, oval-shaped port passes from each blind hole to the
cell chamber, such that the electrode solution ports 601 and 602 open
onto the first electrode recess 608. In this manner, the electrode
solution is allowed to flow into the apparatus through one of the
electrode solution ports and across the electrode (not shown). When the
electrode solution exits the apparatus, it flows across the electrode,
through the angled portion of the other electrode solution port, and then
through the straight portion of that solution port and out of the
apparatus. In an embodiment, electrode solution port 601 is an electrode
solution inlet port, and electrode solution port 602 is an electrode
solution outlet port. In an embodiment, electrode solution port 601 is an
electrode solution outlet port, and electrode solution port 602 is an
electrode solution inlet port. Although an oval-shaped port is shown, it
will be appreciated that any opening shape may be used. Further, it will
be appreciated that any depth short of the depth of the cell recess 606,
including up to the cell recess 606 depth itself, may be used, such that
any angle formed as a result of the selected depth may be used.

[0101] The first axial support member also includes first and second
solution ports 603 and 604. In an embodiment, the first solution port 603
is an acidic solution inlet, allowing acidic solution to enter the
apparatus, and the second solution port 604 is a basic solution inlet,
allowing basic solution to enter the apparatus. In an embodiment, the
first solution port 603 is a basic solution inlet, allowing basic
solution to enter the apparatus, and the second solution port 604 is an
acidic solution inlet, allowing acidic solution to enter the apparatus.
In both embodiments, the solution flow can be reversed such that first
and second solution ports 603 and 604 become solution outlets, allowing
acidic and basic solutions to flow out of the apparatus. In an
embodiment, the first and second solution ports 603 and 604 are formed
substantially on either side of the electrode solution port 601. However,
any other arrangement can be used, so long as it allows for the first and
second solution ports 603 and 604 to substantially align with their
corresponding membrane apertures in the ion exchange membranes (see FIG.
1) such that first and second solutions flow through the apparatus.

[0102] In an embodiment of a high-pressure electrodialysis apparatus that
uses one or more three-compartment BPMED cells, the first axial support
members include an additional, third solution port (not shown). The third
solution port is formed in a position that corresponds with the third
solution port hole of the corresponding reinforcing member. In an
embodiment, the third solution may be, for example, a salt solution that
is diluted upon passing through the operating electrodialysis unit. The
third solution port allows a third solution to enter or exit the
apparatus, depending on the direction of the flow.

[0103] The first axial support member also includes an electrode stem hole
605 to allow the stem of the first electrode to pass through the first
axial support member. The electrode stem hole 605 may be formed, for
example, substantially in the center of the first axial support member.

[0104] In an embodiment, the first axial support member also includes a
cell recess 606 configured to receive the electrodialysis stack. In an
embodiment, the cell recess 606 is formed within the first axial support
member such that the cell recess 606 has a smaller size than the first
axial support member, for example, a smaller diameter. The cell recess
606 is also formed within the first axial support member such that the
cell recess 606 has a larger size than the electrodialysis stack, for
example, a larger diameter. In this way, a gap is created at the
perimeter of the cell recess 606 such that when the first axial support
member and the second axial support member are mated to form the cell
chamber, the electrodialysis stack does not come into contact with the
interior side walls of the cell chamber so that a gap between the stack
and the cell chamber side walls exists. This gap may be any size.

[0105] In an embodiment, the first axial support member also includes one
or more pressurization ports 607. The pressurization ports 607 may be
formed, for example, substantially at the perimeter of the cell recess
606. The pressurization ports 607 allow the pressure between the interior
of the electrodialysis stack and the cell chamber to equalize by
diverting a portion of the electrode solution into the gap between the
electrodialysis stack and the interior cell chamber walls, i.e., the
region of the cell chamber outside the electrodialysis stack. In this
way, a "pressure basket" is formed around the electrodialysis stack. In
an embodiment, the diverted portion of the electrode solution may be a
negligible fraction of the total electrode solution volume. The
pressurization ports 607 may be formed such that they are directly
connected to the electrode solution ports 601 and 602, for example, by
machining a bleed hole from each of the ports 601 and 602. In this way,
the electrode solution ports 601 and 602 can be connected to the gap
between the electrodialysis stack and the interior cell chamber wall,
allowing the electrode solution ports 601 and 602 to communicate with the
cell chamber by transmitting a portion of the electrode solution into the
gap.

[0106] In an embodiment, the first axial support member also includes a
first electrode recess 608 configured to receive the first electrode (not
shown). In an embodiment, the first electrode recess 608 is also
configured to receive the first electrode grate (see FIGS. 9-10). The
first electrode recess 608 is formed within the first axial support
member such that the first electrode recess 608 lies substantially within
the cell recess 606. In an embodiment, the first electrode recess 608 is
deeper than the cell recess 606. In an embodiment, the electrode stem
hole 605 is formed substantially in the center of the first electrode
recess 608 such that the electrode stem passes through the first axial
support member and out the exterior side of the first axial support
member when the electrode (not shown) is received in the first electrode
recess 608. In an embodiment, the electrode rests within the first
electrode recess 608 such that a space is created between the face of the
electrode active surface disk and the electrodialysis cell. In an
embodiment, the electrode grate also rests within the first electrode
recess 608 and is positioned substantially on top of the face of the
electrode active surface disk. In an embodiment, the first electrode
recess 608 is formed to a depth deep enough to receive both the electrode
and the electrode grate such that the face of the electrode grate is
substantially planar with the face of the cell recess 606 when the
electrode and electrode grate are received in the first electrode recess
608.

[0107] In an embodiment, the first electrode recess 608 includes one or
more first electrode recess alignment features 609. The first electrode
recess alignment feature 609 may be, for example, a notch formed in the
perimeter of the first electrode recess 608. The first electrode recess
alignment feature 609 allows the first electrode grate (see FIGS. 9-10)
to be aligned in the first electrode recess 608. For example, an
embodiment first electrode grate may have an electrode grate alignment
feature that is a nib formed substantially on the outer perimeter of the
grate's spacing rim; the first electrode recess alignment feature 609 may
be a groove of corresponding size to the nib such that the nib fits
substantially within the groove when the electrode grate is received in
the first electrode recess 608. Aligning the electrode grate in the first
electrode grate recess 608 facilitates alignment of the grate solution
ports with the electrode solution ports 601 and 602. Although the
embodiment first electrode recess 608 shown in FIG. 6 has only one first
electrode recess alignment feature 609, it will be appreciated that any
number of first electrode recess alignment features 609 may be used.

[0108] In an embodiment, the first axial support member also includes one
or more support member alignment features 610. The support member
alignment features 610 may be formed substantially near the perimeter of
the cell recess 606. The support member alignment features 610 correspond
to support member alignment features in the second axial support member,
as well as to alignment features in the membranes and gaskets of the
electrodialysis stack. The support member alignment features 610 allow
the first axial support member, the second axial support member, and the
membranes and gaskets of the electrodialysis stack to be aligned.
Aligning features, for example pins, are placed in the support member
alignment features 610 to facilitate alignment of the support members and
electrodialysis stack. In an embodiment, the aligning features may be,
for example, stainless steel pins.

[0109] Although the support member alignment features 610 in FIG. 6 are
depicted as holes, it will be appreciated that any shape that facilitates
alignment can be used. Further, although the first axial support member
shown in FIG. 6 has two support member alignment features 610, it will be
appreciated that any number of support member alignment features 610 can
be used.

[0110] The first axial support member also includes a mating face 611. In
an embodiment, the mating face 611 has a groove formed within the mating
face that receives a seal 612. In another embodiment, the mating face 611
may not have a groove for receiving the seal 612. The seal 612 may be,
for example, a Viton® fluoroelastomer o-ring. The groove may be
formed near the inner perimeter of the mating face 611 such that the seal
612 is located near the inner perimeter of the mating face 611. The seal
612 allows the mating face 611 to be selectively coupled to the mating
face of the second axial support member such that the cell chamber is
formed. In an embodiment, when the first and second axial support members
are mated, the cell chamber has a depth such that when seven
electrodialysis cells are used, there is approximately a 10% squeeze on
the gaskets, ensuring good cell-to-cell and cell-to-chamber seal. In
other embodiments, the squeeze on the gaskets may from 5% to 20%. To
accommodate a lesser number of cells, appropriately thick "dummy" cells
made, for example, from gasket material and spacer material or stainless
steel and spacer material, may be used instead of an actual cell. The
apparatus can be adapted to receive any number of cells, including more
than seven cells.

[0111]FIG. 7 shows a substantially planar view of the interior face of a
second axial support member according to an embodiment. The elements
701-705 in FIG. 7 are substantially the same as elements 601-605 in FIG.
6 except that they are elements of a second axial support member, rather
than elements of a first axial support member. They are also
substantially the same as elements 501-505 seen in FIG. 5 except that
they are the elements as seen from the interior face, rather than the
exterior face, of a second axial support member.

[0112] The second axial support member includes electrode solution ports
701 and 702. Although electrode solution port 701 is obscured in FIG. 7,
it will be appreciated that in an embodiment, electrode solution port 701
is substantially the same configuration as electrode solution port 702.
The electrode solution ports 701 and 702 allow the electrode solution to
enter and exit the apparatus. In an embodiment, the electrode solution
ports 701 and 702 are formed in the second axial support member such that
they are substantially directly across from each other. However, any
other arrangement can be used, so long as the electrode solution port
holes of the second reinforcing member (see FIGS. 3-4) are formed in
positions that substantially align with their respective electrode
solution ports 701-702.

[0113] In an embodiment, the electrode solution ports 701 and 702 do not
pass straight through the entire thickness of the second axial support
member. Rather, the electrode solution ports 701 and 702 pass from the
exterior side (see FIG. 5) substantially straight through only part of
the thickness of the second axial support member, stopping short of the
mating face 709, forming a "blind" hole at each port. From there, an
angled oval-shaped port passes from each blind hole to the cell chamber,
such that the electrode solution ports 701 and 702 open onto the second
electrode recess 706. In this manner, electrode solution is allowed to
flow into the apparatus through one of the electrode solution ports 701
and across the electrode (not shown). When the electrode solution exits
the apparatus, it flows across the electrode, through the angled portion
of the other electrode solution port, and then through the straight
portion of that electrode solution port and out of the apparatus. In an
embodiment, electrode solution port 701 is an electrode solution inlet
port, and electrode solution port 702 is an electrode solution outlet
port. In an embodiment, electrode solution port 701 is an electrode
solution outlet port, and electrode solution port 702 is an electrode
solution inlet port. Although an oval-shaped port is shown, it will be
appreciated that any opening shape may be used. Further, it will be
appreciated that any depth short of the depth of the mating face 709,
including up to the mating face 709 depth itself, may be used, such that
any angle formed as a result of the selected depth may be used.

[0114] The second axial support member also includes first and second
solution ports 703 and 704. In an embodiment, the first solution port 703
is an acidic solution outlet, allowing acidic solution to exit the
apparatus, and the second solution port 704 is a basic solution outlet,
allowing basic solution to exit the apparatus. In an embodiment, the
first solution port 703 is a basic outlet, allowing basic solution to
exit the apparatus, and the second solution port 704 is an acidic
solution outlet, allowing acidic solution to exit the apparatus. In both
embodiments, the solution flow can be reversed such that first and second
solution ports 703 and 704 become solution inlets, allowing acidic and
basic solutions to flow into the apparatus. In an embodiment, the first
and second solution ports 703 and 704 are formed substantially on either
side of the electrode solution port 702. However, any other arrangement
can be used so long as it allows for the first and second solution ports
703 and 704 to substantially align with their corresponding membrane
apertures in the ion exchange membranes (see FIG. 1) such that first and
second solutions flow through the apparatus.

[0115] In an embodiment of a high-pressure electrodialysis apparatus that
uses one or more three-compartment BPMED cells, the second axial support
members include an additional, third solution port (not shown). The third
solution port is formed in a position that corresponds with the third
solution port hole of the corresponding reinforcing member. In an
embodiment, the third solution may be, for example, a salt solution that
is diluted upon passing through the operating electrodialysis unit. The
third solution port allows a third solution to enter or exit the
apparatus, depending on the direction of the flow.

[0116] The second axial support member also includes an electrode stem
hole 705 to allow the stem of the second electrode to pass through the
second axial support member. The electrode stem hole 705 may be formed,
for example, substantially in the center of the second axial support
member.

[0117] In an embodiment, the second axial support member includes a second
electrode recess 706 configured to receive the second electrode (not
shown). In an embodiment, the second electrode recess 706 is also
configured to receive the second electrode grate (not shown). The second
electrode recess 706 is formed within the second axial support member
such that the second electrode recess 706 lies substantially within the
second axial support member. In an embodiment, the second electrode
recess 706 is deeper than the mating face 709. In an embodiment, the
electrode stem hole 705 is formed substantially in the center of the
second electrode recess 706. In an embodiment, the electrode rests within
the second electrode recess 706 such that a space is created between the
electrode face and the electrodialysis cell. In an embodiment, the
electrode grate also rests within the second electrode recess 706 and is
positioned substantially on top of the electrode face. In an embodiment,
the second electrode recess 706 is formed to a depth deep enough to
receive both the electrode and the electrode grate such that the face of
the electrode grate is substantially planar with the mating face 709 when
the electrode and electrode grate are received in the second electrode
recess 706.

[0118] In an embodiment, the second electrode recess 706 may also include
one or more second electrode recess alignment features 707. The second
electrode recess alignment feature 707 may be, for example, a notch
formed in the perimeter of the second electrode recess 706. The second
electrode recess alignment feature 707 allows the second electrode grate
to be aligned in the second electrode recess 706. For example, an
embodiment second electrode grate may have an electrode grate alignment
feature that is a nib formed substantially on the outer perimeter of the
electrode grate's spacing rim; the second electrode recess alignment
feature 707 may be a groove of corresponding size to the nib such that
the nib fits substantially within the groove when the electrode grate is
received in the second electrode recess 706. Aligning the electrode grate
in the second electrode recess 706 facilitates alignment of the grate
solution ports with the electrode solution ports 701 and 702. Although
the second electrode recess 706 shown in FIG. 7 only has one second
electrode recess alignment feature 707, it will be appreciated that any
number of second electrode recess alignment features 707 may be used.

[0119] In an embodiment, the second axial support member also includes one
or more support member alignment features 708. The support member
alignment features 708 may be formed substantially near the inner
perimeter of the mating face 709. The support member alignment features
708 correspond to support member alignment features in the first axial
support member, as well as alignment features in the membranes and
gaskets of the electrodialysis stack. The support member alignment
features 708 allow the first axial support member, the second axial
support member, and the membranes and gaskets of the electrodialysis
stack to be aligned. Aligning features, for example pins, may be placed
in the support member alignment features 708 to facilitate alignment of
the support members and electrodialysis stack. In an embodiment, the
aligning features may be, for example, stainless steel pins.

[0120] Although the support member alignment features 708 in FIG. 7 are
depicted as holes, it will be appreciated that any shape that facilitates
alignment can be used. Further, although the second axial support member
shown in FIG. 7 has two support member alignment features 708, it will be
appreciated that any number of support member alignment features 708 can
be used.

[0121] The second axial support member also includes a mating face 709.
The mating face 709 can be selectively coupled to the mating face of the
first axial support member via a seal received in a groove formed in the
first axial support member (see FIG. 6). In an embodiment, the groove may
be alternatively formed in the second axial support member such that the
seal for selectively coupling the first and second axial support members
is received in the second axial support member instead. In an embodiment,
the seal may be attached to the second axial support member rather than
the first axial support member, but not received in a groove.

[0122] In an embodiment, the second axial support member does not include
a cell recess. In an embodiment, the second axial support member does not
include a groove to receive a seal. In an embodiment, the second axial
support member does not include at least one pressurization port.

[0123] In an embodiment, the first and second axial support members shown
in FIGS. 5-7 may be formed from PVC. PVC has a tensile strength of
approximately 400 atm (5900 psi). Since the reinforcing members provide
support in the axial direction, the main issue for the axial support
members is hoop stress in the cell chamber wall formed when the first and
second axial support members are coupled together. As an example, Lame's
thick walled cylinder equations applied to the wall of the first axial
support member from the outside of the first axial support member
(diameter 300 mm) to the outer wall of the groove (252.55 mm), at a
pressure of 20 atm (300 psi), show a maximum hoop stress of approximately
118 atm (1760 psi), with a safety factor of 3.35. This is pessimistic
because the first axial support member may only be this thin for the
depth of the o-ring groove, and in reality would receive support from its
neighboring thicker wall, and since the cylinder is shallow compared to
its diameter, the back wall helps provide support as well.

[0124] A Solidworks FEA simulation for PVC was also conducted where
pressure was applied to the entire interior, i.e., wetted surfaces of the
cell chamber. A fixed restraint was placed at one of the solution inlet
port faces, where the flange adapter seals, for example, with an o-ring.
Because this is a fixed restraint, the simulation produced higher
stresses at these points than would occur with an actual connection. A
sliding restraint was used on the back and front sides sealing faces,
since movement in the axial direction is constrained by the supporting
members. At a simulated pressure of 10 atm (150 psi), the stress near the
pressurization port of the first axial support member was approximately
43 atm (630 psi), with a safety factor of 9.36. At a simulated pressure
of 20 atm (300 psi), stress near the pressurization port of the first
axial support member was approximately 115 atm (1690 psi), with a safety
factor of 3.49. These safety levels were deemed satisfactory.

[0125] FIG. 8 shows an exterior view of an electrode according to an
embodiment. In an embodiment, first and second electrodes are
substantially the same and are not discussed separately.

[0126] The electrode includes an active surface disk 801, the underside of
which comes substantially in contact with the electrode grate recess (see
FIGS. 6-7) when the electrode is received in the electrode grate recess.
The electrode also includes an electrode stem 802. The electrode stem 802
passes through the axial support member from its interior side through
the electrode stem hole (see FIGS. 6-7), where it may be sealed, for
example, with a seal 803 such as a Viton® fluoroelastomer static male
o-ring. The electrode stem 802 also passes through the reinforcing member
from its interior side when the reinforcing member and axial support
member are coupled together to form a housing section. The electrode stem
802 may have a groove to receive the seal 803. The electrode stem 802
allows electrical connection via the connector 804. The connector 804 may
be, for example, a female banana plug style connector. In an embodiment,
the electrode also includes a joint 805 where the electrode stem 802 is
attached to the active surface disk 801. The electrode may be formed, for
example, from nickel. In an embodiment, the electrode may be, for
example, a Series 200 nickel electrode.

[0127]FIG. 9 shows a substantially planar view of the interior face,
i.e., the face that faces the interior of the cell chamber, of an
electrode grate according to an embodiment. In an embodiment, the first
and second electrode grates are substantially the same and are not
discussed separately.

[0128] The electrode grate includes an electrode grate face 901. The
interior face of the embodiment electrode grate faces the interior of the
cell chamber such that the electrode grate face 901 faces the
electrodialysis stack. The electrode grate face 901 includes a plurality
of exposure apertures 902, which allow the electrode solution to flow
through the electrode grate. Although the exposure apertures 902 shown in
FIG. 9 are holes, it will be appreciated that any number of apertures of
any size, shape, and arrangement that allow the electrode solution to
flow through the electrode grate may be used.

[0129] The electrode grate also includes a spacing rim 903, which allows
the electrode grate face 901 to stand substantially off of the electrode
active disk surface (see FIG. 8). The spacing rim 903 may be formed
substantially on the underside of the electrode grate face 901. The
spacing rim 903 includes grate electrode solution ports 904 formed in the
spacing rim 903 in a corresponding position to the solution inlet and
outlet ports of the axial support members/reinforcing members (see FIGS.
3-7). The grate electrode solution ports 904 allow the electrode solution
to flow into and out of the space created beneath the electrode grate
face 901 by the spacing rim 903.

[0130] In an embodiment, the electrode grate also includes at least one
electrode grate alignment feature 905. The electrode grate alignment
feature 905 may be, for example, a nib formed substantially on the outer
perimeter of the spacing rim 903 that substantially fits into the
corresponding electrode recess alignment feature of the axial support
member (see FIGS. 6-7), for example, a groove. The grate alignment
feature 905 facilitates alignment of the electrode grate in the electrode
grate recess so that the grate solution ports 904 are substantially
aligned with the solution inlet and outlet ports of the axial support
member. Although the embodiment electrode grate shown in FIG. 9 only has
one electrode grate alignment feature 905, it will be appreciated that
any number of electrode grate alignment features 905 may be used.

[0131]FIG. 10 is a substantially planar view of the exterior face, i.e.,
the face that faces the active surface disk of the electrode, of an
electrode grate according to one embodiment. The first and second
electrode grates in an embodiment are substantially the same and will not
be discussed separately.

[0132] In an embodiment, the electrode grate may include an electrode
grate face 1001, a plurality of exposure apertures 1002, a spacing rim
1003, grate solution ports 1004, and an electrode grate alignment feature
1005. These features are all substantially the same as elements 901-905
seen in FIG. 9 except that they are the features as seen from the
exterior face, rather than the interior face, of an electrode grate. As
such, the discussion above with regard to features 901-905 may be
referred to for features 1001-1005.

[0133] In an embodiment, the electrode grate also includes at least one
flow diverting element 1006, which promotes swirling of the electrode
solution as it passes over the electrode and electrodialysis stack,
helping to ensure a more complete reaction. The flow diverting elements
1006 may be formed substantially on the underside of the electrode grate
face 1001.

[0134] A function of the embodiment electrode grate shown in FIGS. 9-10 is
to provide sufficient support in the space between the electrode active
surface and the electrodialysis stack while directing the electrode
solution toward the end gasket such that it is received in the end gasket
opening. It will be appreciated that any element that can perform at
least this function may be used.

[0135] In an embodiment, the electrode grate shown in FIGS. 9-10 is formed
from CPVC. The electrode grate may be formed from a single piece of
material, for example, by a mold.

[0136] Turning to FIG. 11, an exploded view of an electrodialysis cell
according to an embodiment is shown. The electrodialysis cell 1105
includes a first cell gasket 1101, a first ion exchange membrane 1102, a
second cell gasket 1103, and a second ion exchange membrane 1104. The
first and second cell gaskets 1101 and 1103 alternate with the first and
second ion exchange membranes 1102 and 1104 in an axial direction such
that a first cell gasket-first ion exchange membrane-second cell
gasket-second ion exchange membrane sandwich is formed. The first ion
exchange membrane 1102 is substantially interposed between the first and
second cell gaskets 1101 and 1103, and the second cell gasket 1103 is
substantially interposed between the first and second ion exchange
membranes 1102 and 1104.

[0137] In an embodiment, the electrodialysis cell 1105 is a
two-compartment cell configured to carry out BPMED. The first ion
exchange membrane 1102 of the cell 1105 may be, for example, a BPM, and
the second ion exchange membrane 1104 of the cell 1105 may be, for
example, an AEM. Alternatively, the first ion exchange membrane 1102 of
the cell may be, for example, a BPM, and the second ion exchange membrane
1104 of the cell may be, for example, a CEM. It will be appreciated that
other pairings and arrangements of membrane types are possible, depending
on the input solutions used and the desired output solution. In another
embodiment, the electrodialysis cell 1105 is configured to carry out
electrodialysis without bipolar membranes. The first ion exchange
membrane 1102 of the cell may be, for example, an AEM, and the second ion
exchange membrane 1104 of the cell may be, for example, a CEM. Other
configurations of membrane types are also possible for cells 1105
configured to carry out electrodialysis without bipolar membranes,
depending on the input solutions used and the desired output solution.

[0138] The ion exchange membranes 1102 and 1104 each include a plurality
of membrane apertures 1106. The membrane apertures 1106 align either with
the cell gasket openings 1107 or the gasket apertures 1108 of the cell
gaskets 1101 and 1103 to form either the first solution or second
solution loops. In an embodiment where the cell is configured to carry
out either electrodialysis without bipolar membranes or two-compartment
BPMED, the ion exchange membranes 1102 and 1104 each have four membrane
apertures 1106. For each ion exchange membrane 1102 and 1104, the four
membrane apertures 1106 serve as a conduit for first solution inlet,
depicted by dashed line 1114; second solution inlet, depicted by dashed
line 1113; first solution outlet, depicted by dashed line 1116; and
second solution outlet, depicted by dashed line 1115. The flows of each
line 1113-1116 may be reversed if desired. In an embodiment, the first
solution is a basic solution and the second solution is an acidic
solution. In an embodiment, the first solution is an acidic solution and
the second solution is a basic solution.

[0139] For example, in an embodiment, first ion exchange membrane 1102 is
a BPM, and second ion exchange membrane 1104 is an AEM such that the cell
gasket opening 1107 of the first cell gasket 1101 defines a
base-receiving compartment, and the cell gasket opening 1107 of the
second cell gasket 1103 defines an acid-receiving compartment. Acidic
solution is flowed into the apparatus at the anode end 1111 and through
the electrodialysis cell 1105 via a gasket aperture 1108 in the first
cell gasket and a membrane aperture 1106 in the BPM 1102; it is received
in the acid-receiving compartment 1107 of cell gasket 1103; and it
continues to flow through the electrodialysis cell 1105 via a membrane
aperture 1106 in the AEM 1104, as depicted by dashed line 1113. Acidic
solution is flowed out of the electrodialysis cell 1105 through the
cathode end 1112 via a different series of apertures and cell gasket
openings, as depicted by dashed line 1115. Basic solution that is flowed
into the apparatus at the anode end 1111 and through the electrodialysis
cell 1105 is first received in the base-receiving compartment 1107 of
cell gasket 1101; it then flows through a membrane aperture 1106 in the
BPM 1102, a gasket aperture 1108 in the second cell gasket 1103, and a
membrane aperture 1106 in the AEM 1104, as depicted by dashed line 1114.
Basic solution is flowed out of the electrodialysis cell 1105 at the
cathode end 1112 via a different series of apertures and cell gasket
openings, as depicted by dashed line 1116. All four flow patterns may be
reversed if the flow of solutions through the apparatus is reversed.

[0140] In an embodiment, the cell gasket openings 1107 are
convexly-shaped. This shape allows the cell gasket openings 1107 to be
simultaneously aligned with two membrane apertures 1106 on an ion
exchange membrane corresponding to the inlet and outlet apertures for the
same solution. For example, the cell gasket opening 1107 of the second
cell gasket 1103 is simultaneously aligned with two membrane apertures
1106 on both the first ion exchange membrane 1102 and the second ion
exchange membrane 1104. In this way, the cell gasket opening 1107 of the
second cell gasket 1103 defines a solution-receiving compartment between
the two ion exchange membranes 1102 and 1104.

[0141] The orientation of the cell gasket openings 1107 on the cell
gaskets 1101 and 1103 determines whether acidic or basic solution flows
through the cell gasket openings 1107 of the cell gaskets 1101 and 1103.
In an embodiment, the cell gasket opening 1107 on cell gasket 1103 is
oriented to receive solution from the two membrane apertures 1106 not
aligned with the cell gasket opening 1107 on cell gasket 1101, resulting
in different solutions flowing through the cell gasket openings 1107 of
cell gaskets 1101 and 1103.

[0142] In an embodiment, spacers (not shown) are used to substantially
fill the cell gasket openings 1107 of the cell gaskets 1101 and 1103. The
spacers may be, for example, polypropylene mesh, such as, for example,
XN-4820 netting made by Industrial Netting. In an embodiment, the spacers
are cut to a size that substantially fills the cell gasket openings 1107.
The spacers may have a thickness that is approximately the same thickness
as the cell gaskets 1101 and 1103, for example, 0.031'', and may have a
nominal hole size of approximately 0.1'' and an open percent of
approximately 76%. The spacers prevent the ion exchange membranes 1102
and 1104 from touching each other. They also optimize the mass transport
of ions to the adjacent membrane faces by distributing the flow of
solutions across the adjacent membrane surfaces.

[0143] The cell gaskets 1101 and 1103 also include a plurality of gasket
apertures 1108. In an embodiment where the cell is configured to perform
either electrodialysis without bipolar membranes or two-compartment
BPMED, cell gaskets 1101 and 1103 each have two gasket apertures 1108.
The gasket apertures 1108 may be formed in the cell gaskets 1101 and 1103
substantially directly across from each other and at an approximately 90
degree angle to the axis formed by the tips of the convexly-shaped cell
gasket opening 1107. For each cell gasket 1101 and 1103, the gasket
apertures 1108 allow the solution not flowing through the cell gasket
opening 1107 of that specific cell gasket to continue flowing to the rest
of the gaskets and membranes in the apparatus.

[0144] In an embodiment, the convexly-shaped cell gasket opening 1107 is
substantially football-shaped. This modified "football" flow design
pattern of the gaskets minimizes the flow dead zones. A dead zone is
defined as an area where the local flow velocity is much less than the
flow velocity averaged over the entire gasket surface area. For example,
computational fluid dynamics modeling was used to calculate and compare
the flow patterns of commercially available gaskets and spacers with
rectangular cell gasket openings used in Ameridia's EUR2c-7-Bip membrane
stack/electrodialysis cell, with gaskets according to an embodiment that
use the modified "football" design gasket opening. For the baseline cases
of the Ameridia gasket, there were noticeable flow dead zones in corners
of the solution-receiving compartment and areas between the jets. In
contrast, the "football" design only had a thin boundary layer along the
outside wall with reduced flow velocity.

[0145] Although the cell gasket openings 1107 shown in FIG. 11 are
substantially football shaped, it will be appreciated that other
convexly-shaped openings that allow for simultaneous alignment of the
cell gasket opening 1107 with two different membrane apertures 1106 of an
exchange membrane may be used. Further, other convexly-shaped openings
that minimize the dead zones in the flow may be used. For example, a
smoothed diamond-shaped or other rhombus-shaped opening may be used.

[0146] In an embodiment, the ion exchange membranes 1102 and 1104 include
one or more membrane alignment features 1109, and the cell gaskets 1101
and 1103 include one or more gasket alignment features 1110. The membrane
alignment features 1109 may be formed substantially near the perimeter of
the ion exchange membranes 1102 and 1104. The gasket alignment features
1110 may be formed substantially near the perimeter of the cell gaskets
1101 and 1103. The membrane alignment features 1109 and gasket alignment
features 1110 correspond both to each other and to support member
alignment features in the first and second axial supports (see FIGS.
6-7), as well as alignment features in any end ion exchange membranes,
end gaskets (see FIG. 15), or additional cell gasket incorporated in the
stack. The membrane alignment features 1109 and gasket alignment features
1110 allow the first and second cell gaskets 1101 and 1103 and first and
second ion exchange membranes 1102 and 1104 to be aligned with and
connected to each other, as well as to the axial support members and, if
present, the end gaskets, end ion exchange membranes, and/or additional
cell gasket. In an embodiment, aligning features such as dowel pins may
inserted through the membrane and gasket alignment features 1109 and 1110
as well as the alignment features of any end gaskets, end ion exchange
membranes, and/or additional cell gasket, and then inserted in the
alignment features of the axial support members to facilitate alignment.
The dowel pins may be, for example, stainless steel.

[0147] Although the membrane and gasket alignment features 1109 and 1110
in FIG. 11 are depicted as holes, it will be appreciated that any shape
that facilitates alignment can be used. The membrane and gasket alignment
features 1109 and 1110 may be formed substantially at the edges of the
membranes and gasket respectively, for example as notches, such that they
are not enclosed on every side by the material of the membranes or
gasket. Further, although the ion exchange membranes 1102 and 1104 and
cell gaskets 1101 and 1103 shown in FIG. 11 each have two alignment
features, it will be appreciated that any number of alignment features
can be used.

[0148]FIG. 12 is an exploded view of an electrodialysis cell according to
another embodiment. The electrodialysis cell 1207 includes a first cell
gasket 1201, a first ion exchange membrane 1202, a second cell gasket
1203, a second ion exchange membrane 1204, a third cell gasket 1205, and
a third ion exchange membrane 1206. The cell gaskets 1201, 1203, and 1205
alternate with the ion exchange membranes 1202, 1204, and 1206 in an
axial direction such that a first cell gasket-first ion exchange
membrane-second cell gasket-second ion exchange membrane-third cell
gasket-third ion exchange membrane sandwich is formed.

[0149] In an embodiment, electrodialysis cell 1207 is a three-compartment
cell configured to carry out BPMED. The first ion exchange membrane 1202
of the cell 1207 may be, for example, a BPM, the second ion exchange
membrane 1204 of the cell 1207 may be, for example, an AEM, and the third
ion exchange membrane 1206 of the cell 1207 may be, for example, a CEM.
Other arrangements of membrane types may also be possible, depending on
the input solutions used and the desired output solution.

[0150] The ion exchange membranes 1202, 1204, and 1206 each include a
plurality of membrane apertures 1208. In an embodiment, each membrane
1202, 1204, 1206 includes six membrane apertures 1208, two each for each
of three solution loops. The membrane apertures 1208 align with either
the cell gasket openings 1209 or the gasket apertures 1210 of the
adjacent cell gaskets to form either the first solution loop, second
solution loop, or the third solution loop. For each ion exchange membrane
1202, 1204, and 1206, the six membrane apertures 1208 serve as a conduit
for first solution inlet, depicted by dashed line 1215; first solution
outlet, depicted by dashed line 1216; second solution inlet, depicted by
dashed line 1217; second solution outlet, depicted by dashed line 1218;
third solution inlet, depicted by dashed line 1219; and third solution
outlet, depicted by dashed line 1220. The flows of each line 1215-1220
may be reversed if desired. In an embodiment, the first solution is an
acidic solution, the second solution is a basic solution, and the third
solution is a salt solution.

[0151] For example, in an embodiment, first ion exchange membrane 1202 is
a BPM, second ion exchange membrane 1204 is an AEM, and third ion
exchange membrane 1206 is a CEM; cell gasket opening 1209 of the first
cell gasket 1201 defines a base-receiving compartment, cell gasket
opening 1209 of the second cell gasket 1203 defines an acid-receiving
compartment, and cell gasket opening 1209 of the third cell gasket 1205
defines a salt solution receiving compartment. Basic solution is flowed
into the apparatus at the anode end 1213 and is received in the cell
gasket opening 1209 of the first cell gasket 1201; it then flows through
the rest of the cell via a series of membrane apertures 1208 in the BPM
1202, AEM 1204, and CEM 1206, and gasket apertures 1210 in the gaskets
1203 and 1205, as depicted by dashed line 1215. Acidic solution is flowed
into the apparatus at the anode end 1213 and through the electrodialysis
cell 1207 via a gasket aperture 1210 in the first cell gasket 1201 and a
membrane aperture 1208 in the BPM 1202; it is received in the
acid-receiving compartment of the cell gasket 1203; and it continues to
flow through the electrodialysis cell 1207 towards the cathode end 1214
via a series of membrane apertures 1208 in the AEM 1204 and the CEM 1206
and gasket apertures 1210 in gasket 1205, as depicted by dashed line
1217. Salt solution is flowed into the apparatus at the anode end 1213
via a series of gasket apertures 1210 and membrane apertures 1208 in the
first and second cell gaskets 1201 and 1203 and the BPM and AEM 1202 and
1204; it is received in the salt-receiving compartment of the third cell
gasket 1205; and then continues to flow through the cell toward the
cathode end 1214 via a membrane aperture 1208 in the CEM 1206. All three
solutions are flowed out of the cell and out of the apparatus at the
cathode end 1214 through a different series of apertures and gasket
openings, as depicted by dashed lines 1216, 1218, and 1220. All six flow
patterns may be reversed if the flow of the solutions through the
apparatus is reversed.

[0152] In an embodiment, the cell gasket openings 1209 are substantially
convexly-shaped, as described above with respect to the embodiment shown
in FIG. 11. The convexly-shaped opening may be substantially football
shaped. Similar to the embodiment shown in FIG. 11, the orientation of
the cell gasket openings 1209 of the cell gaskets 1201, 1203, and 1205
determines whether acid, base, or salt solution flows through the given
cell gasket opening. For example, in the embodiment shown in FIG. 12, the
first cell gasket 1201 is rotated to a first angle such that its opening
1209 receives a first solution, for example, a basic solution; the second
cell gasket 1203 is rotated to a second angle such that its opening 1209
receives a second solution, for example, an acidic solution; and the
third cell gasket 1205 is rotated to a third angle such that its opening
1209 receives a third solution, for example, a salt solution. Each of the
cell gasket openings 1209 may be substantially filled with a spacer, as
described above with respect to FIG. 11.

[0153] In an embodiment, the cell gaskets 1201, 1203, and 1205 each have a
plurality of gasket apertures 1210. In an embodiment, each element
1201-1206 of the electrodialysis stack 1207 has at least one alignment
feature 1211 and 1212, as described above with respect to FIG. 11.

[0154] FIGS. 13-14 show planar views of the membranes and gaskets seen in
FIG. 11. The membranes and gaskets seen in FIG. 12 are substantially the
same, except that the exchange membrane (FIG. 13) has six membrane
apertures 1302, and the cell gasket (FIG. 14) has four gasket apertures
1402.

[0155]FIG. 13 shows a planar view of an ion exchange membrane according
to an embodiment. The ion exchange membrane has a membrane surface 1301.
It also includes a plurality of membrane apertures 1302. In an embodiment
where the cell is configured to perform electrodialysis without bipolar
membranes or two-compartment BPMED, an ion exchange membrane includes
four membrane apertures 1302: two membrane apertures 1302 for the first
solution loop, with one aperture serving as a first solution input and
the other as an first solution output; and two membrane apertures 1302
for the second solution loop, with one aperture serving as a second
solution input, and the other as a second solution output. In an
embodiment where the cell is configured to perform three-compartment
BPMED, an ion exchange membrane includes six membrane apertures 1302: two
for the first solution loop, two for the second solution loop, and two
for the third solution In an embodiment, the ion exchange membrane may
also include one or more membrane alignment features 1203 to facilitate
alignment of the membrane with other components of the apparatus.

[0156] The ion exchange membrane may be either an end ion exchange
membrane, a first ion exchange membrane of a cell, or a second ion
exchange membrane of a cell. In an embodiment, the end ion exchange
membrane may be, for example, a cation exchange membrane. In an
embodiment, the end ion exchange membrane may be, for example, an anion
exchange membrane. In an embodiment of a high-pressure two-compartment
bipolar-membrane electrodialysis apparatus, the first ion exchange
membrane and the second ion exchange membrane may be, for example, a
bipolar exchange membrane and an anion exchange membrane, respectively.
In another embodiment of a high-pressure two-compartment bipolar-membrane
electrodialysis apparatus, the first ion exchange membrane and the second
ion exchange membrane may be, for example, a bipolar exchange membrane
and a cation exchange membrane, respectively. In another embodiment of a
high-pressure two-compartment electrodialysis apparatus, the first ion
exchange membrane and the second ion exchange membrane may be, for
example, an anion exchange membrane and a cation exchange membrane,
respectively. In another embodiment of a high-pressure three-compartment
bipolar-membrane electrodialysis apparatus, the first ion exchange
membrane and the second ion exchange membrane and the third ion exchange
membrane may be, for example, a bipolar exchange membrane and a anion
exchange membrane and a cation exchange membrane, respectively.

[0157]FIG. 14 shows a planar view of a cell gasket, both those used in
the electrodialysis cell and the additional cell gasket used in the
stack, according to one embodiment. The cell gasket includes a cell
gasket opening 1401 that defines a solution-receiving compartment. In an
embodiment, a spacer (not shown) is inserted in the cell gasket opening
1401.

[0158] The cell gasket also includes a plurality of gasket apertures 1402.
In an embodiments where the cell is configured to perform either
electrodialysis without bipolar membranes or two-compartment BPMED, the
cell gasket includes two gasket apertures 1402 formed in the gasket
substantially directly across from each other and at an angle to the axis
formed by the ends of the cell gasket opening 1401. This angle may be,
for example, approximately 90 degrees. If the cell gasket opening 1401
defines a first solution-receiving compartment, then the gasket apertures
1402 are both for the second solution loop, with one aperture serving as
a second solution input, and the other as a second solution output. If
the cell gasket opening 1401 defines a second solution-receiving
compartment, then the gasket apertures 1402 are both for the first
solution loop, with one aperture serving as an first solution input, and
the other as an first solution output. In an embodiment where the cell is
configured to perform three-compartment BPMED, the cell gasket includes
four gasket apertures 1402. If the cell gasket opening 1401 defines a
first-solution receiving compartment, then two gasket apertures 1402 are
for the second solution loop, and two are for the third solution loop. If
the cell gasket opening 1401 defines a second-solution receiving
compartment, then two gasket apertures 1402 are for the first solution
loop, and two are for the third solution loop. If the cell gasket opening
1401 defines a third-solution receiving compartment, then two gasket
apertures 1402 are for the first solution loop, and two are for the
second solution loop.

[0159] In an embodiment, the cell gasket also includes one or more gasket
alignment features 1403 to facilitate alignment of the gasket with other
components of the apparatus.

[0160]FIG. 15 shows a planar view of an end gasket configured to be used
in an electrodialysis stack that includes at least one cell for
electrodialysis without bipolar membranes or at least one two-compartment
BPMED cell (see FIG. 11). An end gasket configured to be used in an
electrodialysis stack that includes at least one three-compartment BPMED
cell is substantially the same, except that it has six gasket apertures
1502.

[0161] The end gasket includes an end gasket opening 1501. In an
embodiment, a spacer (not shown) is inserted in the cell gasket opening
1501. The spacer may be, for example, polypropylene mesh, such as, for
example, XN-4820 netting made by Industrial Netting. In an embodiment,
the spacer is cut to a size that substantially fills the end gasket
opening 1501. The spacer may have a thickness that is approximately the
same thickness as the end gaskets, for example, 0.031'', and may have a
nominal hole size of approximately 0.1'' and an open percent of
approximately 76%. The spacer prevents ion exchange membranes adjacent to
the end gasket from touching each other. It also optimizes the mass
transport of ions to the adjacent membrane faces by distributing the flow
of solutions across the adjacent membrane surfaces.

[0162] In an embodiment, the end gasket opening 1501 is slightly larger
than the electrode grate such that the electrode solution that flows into
the electrode grate is received in the compartment defined by the end
gasket opening 1501. In an embodiment, the end gasket also includes a
plurality of end gasket apertures 1502. In embodiments where the cell is
configured to perform either electrodialysis without bipolar membranes or
two-compartment BPMED, the end gasket includes four end gasket apertures
1502: two for the first solution loop, with one aperture serving as a
first solution input and the other as a first solution output; and two
for the second solution loop, with one aperture serving as a second
solution input, and the other as a second solution output. In an
embodiment where the cell is configured to perform three-compartment
BPMED, the end gasket includes six end gasket apertures 1502: two for the
first solution loop, two for the second solution loop, and two for the
third solution loop.

[0163] In an embodiment, the end gasket also includes one or more end
gasket alignment features 1503 to facilitate alignment of the end gasket
with other components of the apparatus. The end gasket alignment features
1503 may be formed substantially near the perimeter of the end gasket.
The end gasket alignment features 1503 correspond to the alignment
features of the ion exchange membranes and cell gaskets (see FIGS. 11-14)
and the support member alignment features in the first and second axial
supports (see FIGS. 6-7), as well as the alignment features of any end
ion exchange membranes and additional cell gaskets incorporated in the
stack. The end gasket alignment features 1503 facilitate alignment of the
end gasket with these other components. Dowel pins may be used to
facilitate alignment. The dowel pins may be, for example, stainless
steel.

[0164] Although the end gasket alignment features 1503 in FIG. 15 are
depicted as holes, it will be appreciated that any shape that facilitates
alignment can be used. The end gasket alignment features 1503 may be
formed substantially at the edges of the end gasket, for example as
notches, such that they are not enclosed on every side by the material of
the end gasket. Further, although the end gasket shown in FIG. 15 has two
end gasket alignment features 1503, it will be appreciated that any
number of alignment features can be used.

[0165] High-pressure electrodialysis apparatuses according to embodiments
herein described can be used to perform, for example, two-compartment or
three-compartment high-pressure BPMED of gas-evolving solutions or
high-pressure electrodialysis without bipolar membranes of gas-evolving
solutions. High-pressure electrodialysis of gas evolving solutions is
capable of producing gas at any pressure between ambient pressure and the
absolute pressure of the membrane stack. By pressurizing the liquid in
the membrane stack and then performing ion transport into a solution from
which gas evolves, this gas pressurization is accomplished much more
efficiently than what is typically possible. In the case of CO2,
this allows direct generation of CO2 gas at the pressures required
either for sequestration, fuel reaction, or other applications. This may
also be a benefit for other gases that could be directly generated at the
elevated pressure required for a subsequent reaction step or other
applications.

[0166] By operating at pressures above ambient pressure, including
pressures of 20 atm or greater, a high-pressure electrodialysis apparatus
according to described embodiments can suppress gas bubble evolution
inside the electrodialysis membrane stack, thereby eliminating the
resulting localized "hot spots" of high current density that damage the
membranes. A high-pressure electrodialysis apparatus according to
described embodiments allows electrodialysis of gas-evolving solutions at
higher nominal current densities with improved membrane lifetimes
compared to electrodialysis of the same solutions at ambient pressure.
Also, by operating at pressures above ambient pressure, a high-pressure
electrodialysis apparatus according to described embodiments can achieve
reduced voltage and energy consumption compared to electrodialysis of the
same solutions at ambient pressure. This becomes increasingly true with
increasing nominal operating current density. For example, in an
embodiment where two-compartment BPMED of CO2-evolving bicarbonate
solutions is performed at a current density of 139 mA/cm2, the
energy required per mole of CO2 generated is reduced by 30% when
operating at a pressure of 10 atm relative to operation at 1.5 atm. A
high-pressure electrodialysis apparatus according to described
embodiments can also be viewed as an efficient electrochemical gas
pressurizer that is more efficient than employing standard apparatuses
for pressurizing gas.

[0167] Turning to FIG. 16, an exploded view of an embodiment of a
high-pressure electrodialysis apparatus is shown. In an embodiment, the
apparatus is a high-pressure two-compartment BPMED apparatus that can be
used, for example, to generate CO2 gas from aqueous carbonate and
bicarbonate solutions.

[0168] The high-pressure apparatus includes an anode end 1601 and a
cathode end 1602. During operation, the first electrode 1607 at the anode
end 1601 is held at a positive electrical potential relative to the
electrical potential of the second electrode 1609 at the cathode end
1602. The housing of the high-pressure apparatus includes a first axial
support member 1603 and a first reinforcing member 1604 at the anode end
1601, and a second axial support member 1605 and a second reinforcing
member 1606 at the cathode end 1602. In an embodiment, the first axial
support member 1603 at the anode end 1601 includes at least one
pressurization port 1622. The housing further includes flange adapters
1619 that fit over the bossed ports of the first and second axial support
members 1603 and 1605 when they are slid through the port holes of the
first and second reinforcing members 1604 and 1606. In an embodiment, the
housing may further include stress reduction cylinders 1620, one attached
to each reinforcing member 1604 and 1606.

[0169] The high-pressure apparatus also includes a first electrode 1607
and first electrode grate 1608 at the anode end 1601, and a second
electrode 1609 and second electrode grate 1610 at the cathode end 1602.

[0170] The high-pressure apparatus also includes an electrodialysis stack
1611 that includes a plurality of electrodialysis cells 1612. In an
embodiment, the electrodialysis stack 1611 includes seven electrodialysis
cells 1612; however, embodiments of the high-pressure apparatus can be
adapted to receive any number of electrodialysis cells 1612. The
electrodialysis cell 1612 includes a first cell gasket 1613, first ion
exchange membrane 1614, second cell gasket 1615, and second ion exchange
membrane 1616. In an embodiment, the first ion exchange membrane 1614 is
a BPM, and the second ion exchange membrane 1616 is an AEM. Other
configurations of cells, already discussed, may be alternatively used,
such as three-compartment cells, or electrodialysis without bipolar
membranes.

[0171] In an embodiment, the electrodialysis stack 1611 also includes end
ion exchange membranes 1617, one at either end of the plurality of
electrodialysis cells 1612. In an embodiment, the end ion exchange
membranes 1617 are CEMs; other types of exchange membranes may be used,
depending on the configuration of the stack as well as the input
solutions used and the desired output solution. In an embodiment, the
electrodialysis stack 1611 also includes two end gaskets 1618, one at
either end of the electrodialysis stack 1611. The end gaskets 1618 are
substantially interposed between the end ion exchange membranes 1617 and
the electrode grates 1608 and 1610. In an embodiment, the electrodialysis
stack 1611 also includes an additional cell gasket 1621 substantially
interposed between the nth electrodialysis cell 1612 and the end ion
exchange membrane 1617 at the cathode end 1602 of the electrodialysis
stack 1611. It will be recognized that whether end ion exchange membranes
210 and an additional cell gasket 212 are needed, and what types of ion
exchange membranes 210 are appropriate, depends on the input solutions
used and the desired output solution. In an embodiment, the gasket
openings of the cell gaskets 1612, 1615, and 1621 and the end gaskets
1618 are substantially filled with spacers.

[0172] In an embodiment, the high-pressure apparatus also includes
aligning features (not shown) for aligning the elements of the
electrodialysis stack 1611 to each other and to the first and second
axial support members 1603 and 1605 through their respective alignment
features (shown). The aligning features may be, for example, stainless
steel dowel pins. An embodiment high-pressure apparatus also includes
connecting features (not shown) for aligning and connecting the first and
second reinforcing members 1604 and 1606 to each other through their
respective connection features (shown). The connecting features may be,
for example, a bolted connection such as 10 mm, class 8.8 Hex head cap
screws.

High-Pressure Electrodialysis System

[0173]FIG. 17 is a schematic of an embodiment of a high-pressure
electrodialysis system. The system consists of three different loops: one
for a first solution, one for a second solution, and one for an electrode
solution. In another embodiment, the system consists of four different
loops: one for a first solution, one for a second solution, one for a
third solution, and one for an electrode solution. Although FIG. 17
depicts a system with three loops such that an apparatus for conducting
two-compartment BPMED or electrodialysis without bipolar membranes can be
incorporated in the system, it will be appreciated that the system can be
adapted to instead incorporate a three-compartment BPMED apparatus by
incorporating an additional loop for a third solution.

[0174] In an embodiment, the first solution is a basic solution and the
second solution is an acidic solution. In an embodiment, the first
solution is an acidic solution and the second solution is a basic
solution. An electrode solution may be, for example, an electrolyte. In
an embodiment incorporating four loops, the third solution may be, for
example, a salt solution that is diluted upon passing through the
operating electrodialysis unit.

[0175] Each loop can be operated in either bypass or stack mode. When a
loop operates in bypass mode, the solutions bypass the electrodialysis
membrane stack of the high-pressure electrodialysis apparatus 1710. Each
loop includes one or more tanks (for example, first solution tank 1700),
a pump (for example, first solution pump 1704), a pulsation dampener (for
example, first solution pulsation dampener 1707), one or more temperature
and pH meters (for example, meter 1716), one or more temperature and
conductivity meters (for example, meter 1717), one or more pressure and
flow meters (for example, meters 1723 and 1723), valves to change the
operation mode between bypass mode and stack mode (for example, valves
1712 and 1720), and a valve to adjust the pressure of the electrodialysis
apparatus 1710 (for example, valve 1721).

[0176] In an embodiment, the high-pressure electrodialysis system also
includes a high-pressure electrodialysis apparatus 1710. The
high-pressure electrodialysis apparatus may be any one of the embodiments
already discussed. In an embodiment, the electrodialysis apparatus 1710
includes a BPMED stack such that the electrodialysis system of FIG. 1 can
be used, for example, to generate CO2 gas from aqueous carbonate and
bicarbonate solutions.

[0177] In an embodiment, tanks 1700-1703 of the high-pressure
electrodialysis system are designed to withstand pressures up to
approximately 20 atm with a safety factor of about 3. The tanks 1700-1703
can be pressurized to any pressure between ambient pressure and the
absolute pressure of the electrodialysis stack. When the tanks are so
pressurized, gas evolves out of solution at a pressure above ambient
pressure, i.e., at a pressure substantially the same as the pressure of
the tanks 1700-1703. In an embodiment, the covers of the tanks 1700-1703
have three threaded holes of a diameter of 1/4 inch for optional pipe
connections and a fill cap of a diameter of 11/8 inch to fill the tanks
1700-1703.

[0178] In an embodiment, the first and second solution tanks 1700 and 1701
each have both an inlet coming from the bypass or the electrodialysis
apparatus 1710 and an outlet going to the first solution pump 1704 or
second solution pump 1705 respectively. Both tanks 1700 and 1701 have a
feed-through where the pH and the conductivity meters 1716 and 1717 are
introduced into the solution. The cover of the second solution tank 1701
is equipped with a vent and a 1/8 inch pipe for gas infusion. In an
embodiment, the first solution tank 1700 has two windows to observe the
evolution of gas bubbles from the solution. In an embodiment where the
tank is pressurized, the tank cover may be equipped with a pressure
sensor to know the pressure of the tank. The tank may also be equipped
with two vents: one that leads the gas to a valve and a flow meter in
order to regulate and measure the flow of gas evolving from the solution,
and one to empty the headspace and flush the tank with gas.

[0179] In an embodiment, the first and second solution tanks 1700 and 1701
both have meters for measuring, showing, and recording certain
information about the tank conditions. For example, the first solution
tank 1700 may include temperature and pH information and recording to
computer (TXIR) meter 1716 that measures, shows, and records the
temperature and pH of the first solution. The first solution tank 1700
may also include a TXIR meter 1717 that measures, shows, and records the
temperature and conductivity of the first solution. The second solution
tank 1701 may also have a TXIR meter 1716 and TXIR meter 1717 that
perform the same functions for the second solution.

[0180] In an embodiment, the second solution tank 1701 may also have a
means for infusing gas into the second solution in order to regenerate
the gas while operating through the electrodialysis stack. For example,
in an embodiment electrodialysis system that is used to generate CO2
gas, the second solution tank 1701 may be a base solution tank that has a
CO2 gas bottle connected to it by, for example, a 1/8 inch pipe. The
infusion of gas into the second solution tank 1701 can be turned on or
off using valve 1718.

[0181] In an embodiment, the first solution loop also includes a valve
1719 to drain the first solution loop and a valve 1720 to change the
operation from the bypass to the electrodialysis stack mode. The second
solution loop also includes valves 1719 and 1720 that perform the same
functions with respect to the second solution loop. The first solution
loop also includes a valve 1721 to change the pressure in the
electrodialysis system and a valve 1722 for taking samples. The second
solution loop also includes valves 1721 and 1722 that perform the same
functions with respect to the second solution loop. In an embodiment, the
pressure and the flow of the first solution loop are measured and
recorded by pressure information and recording to computer (PIR) meter
1723 and flow information and recording to computer (FIR) meter 1724,
respectively. The second solution loop also includes PIR meter 1723 and
FIR meter 1724 that perform the same functions with respect to the second
solution loop.

[0182] In an embodiment, the electrode solution loop has two tanks
1702-1703. Each tank 1702-1703 has an inlet coming from the bypass or the
electrodialysis apparatus 1710 and an outlet going to the electrode
solution pump 1706. The electrode solution tank 1702 is for the electrode
solution coming from the anode end of the electrodialysis apparatus from
where oxygen will evolve. The electrode solution tank 1703 is for the
fluid coming from the cathode end of the electrodialysis apparatus from
where hydrogen will evolve. In an embodiment, each tank 1702-1703 is
equipped with a pipe connection that has a 1/4 inch barb fitting to vent
the tanks from the evolving gases. The vent of tank 1703 may be connected
to the building exhaust to avoid high H2 concentrations in the
atmosphere.

[0183] In an embodiment, the electrode solution loop also has a valve 1711
to drain the electrode solution loop. It may also include two valves 1712
to change the operation from the bypass to the electrodialysis stack
mode. It may also include two valves 1713 that can be varied in order to
change the pressure in the electrodialysis system. In an embodiment, the
pressure and flow of the electrode solution loop are measured and
recorded by PIR meter 1714 and FIR meter 1715, respectively.

[0184] In an embodiment, each port of the electrodialysis apparatus 1710
is equipped with a check valve 1728 to avoid the flow of any solution
into the electrodialysis apparatus 1710 while the system operates in the
bypass mode. Each loop has a pulsation dampener 1707 to damp the pressure
oscillation caused by the operation of the positive displacement pumps
1704.

[0185] In an embodiment, the electrodialysis system includes valve 1731, a
three-way valve connected to a house vacuum. Before the electrodialysis
system is used, valve 1731 can be alternately opened and closed with
valve 1718, which is connected to a cylinder of whatever gas the system
is being used to generate, for example, CO2. Doing so purges the
headspace of first solution tank 1700 of air so that the headspace
contains nothing but whatever gas the system is being used to generate,
for example, CO2. Purging the headspace in this manner ensures that
all the gas that flows through FIR meter 1730 is pure, or as close to
pure as possible, which ensures accurate measurements of the gas flow by
FIR meter 1730.

[0186] In an embodiment, the electrodialysis system includes motors 1725,
one motor for each of the first solution, second solution, and electrode
solution loops. It also includes Hz/RPM controllers 1726, one each for
the first solution, second solution, and electrode solution loops. It
also includes pressure relief valves 1727, one each for the first
solution, second solution, and electrode solution loops. It also includes
valve 1732, a two-way valve that allows the operator of the system to
connect or block the flow from the vacuum or gas cylinder; which is
blocked depends on the settings of valve 1731 and valve 1718. It also
includes valve 1733, a needle valve for controlling the gas flow through
FIR meter 1730. The electrodialysis system also includes a drain 1729 to
receive any solution drained from tanks 1700-1703 via their respective
valves 1711 and 1719.

[0187] In an embodiment, the values of each measuring position of the
system are read and recorded every five seconds by computer software.
These values can be saved and recalled when desired. Table 1 summarizes
the measuring positions of one embodiment of a system and their
functions. Other measurement positions not shown on the schematic are
those that measure the voltage and current going through the membrane
stack.

TABLE-US-00001
TABLE 1
Measuring Positions and their Functions
Measuring Position Function Input/Output
FIR meters 1715, 1724 Measure, show, and record Input
the flow of the loop
PIR meters 1714, 1723 Measure, show, and record Input
the pressure of the loop
TXIR meters 1716, 1717 Measure, show, and record Input
the temperature and either
the pH or the conductivity
of the base or acid solution
FIR meter 1730 Measures, shows, and records Input
the flow of the gas evolving
from the acid tank
VFD meters 1726 Record and control the Output
frequency or RPM of the
motor. Builds a control loop
with the FIR to control the
flow of the loop.

[0189] It will be appreciated that variants of the above-disclosed and
other features and functions, or alternatives thereof, may be combined
into many other different systems or applications. Various presently
unforeseen or unanticipated alternatives, modifications, variations, or
improvements therein may be subsequently made by those skilled in the art
which are also intended to be encompassed by the following claims.